Welcome Bex,
6 months isn’t really long. Strumming is one of those things that can take time to master. Firstly you should only compare you six months ago who couldn’t strum a chord to what you can do today. If you keep at it you will get better. I have Justin’s old strumming course that I am working through, still not managed to get through it all, but it is about gaining the skill, not finishing the videos for me. Justin has just released what looks like a more in depth strumming course recently, even the introductory words to the class talk about how challenging strumming can be.
https://www.justinguitar.com/store/strumming-sos
I recently switched back to a thinner pick, which has helped me. Also recording yourself allows you to demonstrate your daily progress to keep you motivated.
Don’t worry too much, 6 months is really no time at all.
The full post by @Richard_close2u below includes an advice on how much time should ideally be spent on the lessons, though individual skill levels may vary, of course. Check it out.
Hi Bex, welcome to the community, good to have you here 
It sounds like you’ve zipped through the courses quickly if you’re on grade 3 in 6 months, I wonder if you’ve really taken much time consolidating with song playing or you’ve more ticked off the modules? Strumming is something that will only come with time I believe, if I think back to how my stumming was after 6 months compared to now it’s night and day, and my strummng is hardly what you’d describe as brilliant now!
@RobDickinson makes the same suggestion I would, zero in a a couple of patterns / songs and really focus on them for every practice, over several weeks. If one or two specific songs play along to the original or a backing track to help you with the rhythm and flow.
Please don’t get dicouraged as you’ve, no doubt, got ability with the progression you’ve had with the course so far. I’d also try not to fall into the trap of comparing yourself too much to others, we are all so different in what we find “easy”, somebody who may strum well could be really struggling with chord changes and the dreaded F for example. Practice will pay, and patience will be rewarded.
All the best 
OK also for context - I’m 51 and played a LOT when younger from 16 to about 24, and restarted basically a year ago and I am STILL on grade 3 By strumming has good and bad points (Its quite musical from a dynamic pov but not as accurate as I’d like.
These things take time!
Strumming SOS grade 1 course really really really helped me. i’ve been playing for a year only recently figured out Strumming.
Besides all the good tips here, you might want to reconsider that assumption 
Your dominant hand should do the the boss of the rhythm.
Your issue isn’t uncommon to people using the non-dominant hand.
You could be righthanded to write but lefthanded for guitar.
Or ambidextrous for some basic stuff like the reflex of grabbing a glass of water.
We have seen various mixed cases here (myself included) so why not go for a test?
Check this thread, we talk about it there
hit a pot with a spoon in a steady rhythm with your left hand and then with your right hand. what felt best?
Sit down and tap your left leg with your left hand and your right leg with your right hand. count 1-2-3-4 and alternate hand so the “1” is left, the 2 is right, the 3 is leftand the 4 is right. keep counting 1-2-3-4-1-2-3-4 etc. What feel like you are in control the most?
Image a ball coming at you
it coming at you real fat!
imagine punching it away an make the movement.
Which hand did you use?
grab a salt or pepper shaker. shake a rhythm.
Count out loud when you do this
1 and a 2 and a 3 and a 4 and a 1 and a 2 …
What hand did you use?
I don’t have much to add to what others have already said, so I’m just supporting what you’ve already been told.
To be on Grade 3 after just six months means that you are either a genius when it comes to learning instruments and building muscle memory, or that you’re skipping through the lessons much too quickly and are not consolidating anything.
Guitar isn’t about filling your head with knowledge with you use to pass a one-off exam and then forget about. It’s about practice, practice, and more practice. It’s about repeating the same things thousands of times until you can do them without thinking. It’s about taking the time to consolidate things piece by piece, so that you have the fundamentals in place before moving onto new techniques that inevitably rely upon them.
I also recently decided my strumming was not up to scratch and spent three months… half of your total guitar journey… focused solely on improving. It worked, and was achieved through daily, focused practice. However, you should not be comparing yourself to me or anyone else. It’s your journey and will take the time it takes, based on your own strengths, weaknesses and dedication.
Just slow it down, consolidate everything you learn, and get comfortable with the idea that guitar is a journey of years… not months.
Hi Bex and welcome!
I know it’s hard, but the first thing I would say is stop comparing yourself to others. Everyone learns at their own pace, so don’t feel like you are “sucking” at something only because you have seen others who seem to not be struggling as much.
Strumming is a very common hurdle for a lot of people- beginners and even more experienced players alike. Just look at the number of posts in the community asking for strumming help. It takes time and practice to really build up the muscle memory to make things second nature. As others have pointed out, 6 months is not a long time to be at grade 3. Justin works a lot of strumming lessons into grades 1 and 2 and it may be worth going back to revisit those and do a bit more consolidation, at least for the strumming stuff.
If you are making your own practice routine, you may want to work in some time to
practice strumming with the strings muted, just until it feels like you are naturally moving along with the pattern in a groove versus thinking about it so much that it becomes robotic, if that makes sense.
Overall, with more time and practice, you can definitely improve. You’ve got this!
Ditto. I’m just not hearing a song when I attempt the few chords I’ve learnt so far. No fluidity
Just to reinforce the importance of not rushing ahead: the following images from A Modern Method for Guitar, Volume 1 by William Leavitt also emphasize, just like Justin, that you are supposed to continually review what you have learned. Learning any skill is not about ticking boxes in the curriculum and then forgetting everything.
Hi everyone, thank you so much for your comments! I appreciate it so much!
The comments about strumming being difficult for many people, and to stop comparing myself to others, and that 6 months isn’t long enough, and that it’s going to take a lot more time and practice really rang true to me and are really helpful in combating the deflated feeling. I think that could be my main problem right now and it’s encouraging to hear that others have had the same problem and it sounds like it’s unlikely that it wouldn’t eventually get better with practice 
.
I think part of my problem is that I gravitate towards songs that are above my level because they are the songs that keep me motivated to keep practicing, but I should probably spend more time on easier songs with straightforward strumming patterns until they become so ingrained that I can start loosening up and “feeling the music” with them. I have spent a lot of time just practicing strumming, repeating patterns over and over with the strings muted while attempting to do something else like watch TV as Justin recommends in one lesson. I guess I should do a lot more of that. Maybe that has contributed to the robotic-ness, but maybe once it’s really second nature I can loosen it up without losing the rhythm, that robotic-ness will fade.
I think LievenDV might be onto something as well, it’s possible I’ve underestimated how much the left-handedness is affecting this. I seem to be over-achieving on my fretting hand. I have no problem with the F chord and can even do A shaped barre chords (although I can’t switch to those in under five seconds lol!), but the strumming with my right hand just hasn’t quite clicked into what sounds like music to me. (strange that fingerstyle is easier and sounds better to me though).
Regarding the comments that I’m moving through the course too quickly, I completely agree with you. I originally planned to do the program by the book, only progressing after I could honestly say I had completed each consolidation as Justin lays out. However, at one point I found my enthusiasm waning and my practice sessions getting shorter. That was the point at which Justin had said in a lesson that the most important thing at that stage was to do whatever it takes to keep picking up the guitar and practicing every day and staying motivated. Basically, he said if it takes moving ahead to stay motivated, then do it. so that’s what I did. I changed my personal plan from “by the book” to one that allows me to keep moving forward to stay motivated, with the full knowledge that I was not actually passing the lessons and fully integrating all of the skills. My intention is to go back to the beginning later and complete each lesson the proper way. There have been things in my life that I started with great enthusiasm but ultimately abandoned, and I made a pact with myself to not let that happen with guitar. So that is my primary goal with guitar at this juncture, just to stay motivated and keep practicing.
Thanks again to everyone! I feel uplifted .
EDIT: The system tells me I’ve reached my daily limit for sending replies back to everyone so I just want to say thanks again and I’m sorry if I wasn’t able to reply to your comment today.
I love this idea of just getting a bit crazy with it and having fun loosening it up!
Thank you for the reminder that strumming is difficult for many people and that I shouldn’t compare my progress to others!
Thank you for that info! I particularly liked the method of being able to play through full songs even if it’s only one strum per bar. It’s been an issue for me, being able to say I can play songs from start to finish.
Bex, there’s nothing against skipping ahead to keep your interest, however don’t have high expectations of yourself confidently doing the later lessons until you have mastered the early ones. Maybe a short attempt at a later lesson followed by the rest of the session on earlier lessons. Caution though that you do not disillusion yourself more by how much you have to learn. Keep picking up the guitar and practice and you will improve.
Playing along with your favourite songs with simple 1 strum per bar to start with and soon as your skills build, you will get more complex strumming. Playing along to the video even simply, fills in the rest and makes you feel musical…
Learn Songs Learn Songs Learn Songs is a really good mantra from Richard which ensures that the journey is fun.
Thanks so much for the note about your current strumming compared with earlier. It’s important to remember to only compare my progress to myself and how much I’ve learned since the beginning. I remember attempting to shape my first chord (my fingers were sliding all over the place lol!). And it’s helpful to remember that we all have different areas where we excel and where we struggle.
Thank you! It’s encouraging to hear that even a more experienced player struggles with these things. And it’s always nice to hear from another adult learner (I’m 45)
Thank you! It’s encouraging to hear that others have this struggle and that it’s all about practice and time. I’m considering Justin’s strumming course as well!
Thanks! I agree I’m probably underestimating the effect that my left-handedness could be having on this struggle. Maybe I don’t want to think about it lol because I really hemmed and hawed about which way to start learning, I took a bunch of handedness tests and even had a guitar teacher assess and give me advice (he thought I looked more natural pretending to strum with my right hand). Although whenever I’d “air guitar” it was always left handed. I guess what ultimately pushed me towards right handed was a lot of talk on Reddit and other communities about why left handed ppl might prefer to go right handed. Some reasons being that everything is made for righties, chord diagrams, guitars, etc. And once you learn to play and end up at a party or camping and someone whips out a guitar 99 times of 100 it will be a righty so you’re out of luck joining in. And many guitars aren’t even made lefty. So I made my choice. I can’t say I don’t sometimes regret it though! 
Thanks so much! I agree with what you and others have said, it’s just going to take a lot more focused practice