I’ve been trying to learn guitar on and off for years. I feel like I’ll always struggle with chord changes. I’m trying to keep my strumming hand moving at all times but the chord changes are so slow sometimes and they also sound terrible from trying to move too quickly.
Also, I’m struggling to hold my guitar in place when I’m playing it. Any tips for more stability?
Use a strap. Even when sitting.
Hi Aarin, You’re posting this in Grade 2, so I’m guessing you’ve gone through all the Grade 1 lessons. When I got to Grade 2 I had some frustrations as well. It’s worthwhile to go back and selectively review topics. For example, Justin addresses holding the guitar in this lesson as well as this one called Holding the Guitar. For chord changes, there’s the One Minute Changes exercise and this lesson on changing chords while continuing to strum. Go back and review those, start slowly, and increase speed gradually! Oh, and I second @markr31’s guidance to always use a strap.
Following on from @markr31 good advice, the strap should support the guitar not just stop it slipping off your leg, which it shouldn’t, good advice is it should be adjusted so if you stand up it is in the same position as sitting. I use a guitar foot support to raise my leg under the guitar, a teacher advised this, you don’t need it high, if your sitting in a chair with your lower legs at right angles to your thighs the you should only need to raise the supporting leg by about 5cm. These work well Foot rests
On the chord change speed issue, try grabbing the first chord, strum once to ensure you have fingers on strings correctly and it sounds good, take hand off fret board and shake hand out. Repeat this and gradually increase the speed you grab the chord until it is automatic and reasonably quick. Do the same with the other chord. Then practice grabbing the 1st and moving to the 2nd in the same way, once you can change quickly try a simple strum pattern D D D D or D U D U D U D U say at 60bpm and see if you can change ok, if not slow it down and then gradually increase. This approach is what most have to do when learning a new chord or using several in a different sequence. If you are learning a song with 3 or 4 chords, do it for each and each change. I know it sounds tedious but it will be quicker overall.
If you are stumbling on the change overs the try counting out loud to the rhythm to enforce when you need to change, and think about using a metronome.
I am pretty sure Justin talks about most of this in the early lessons, but it’s easy to miss aspects. I remember the 1 minute changes aiming to help with this.
This is the main problem. If you leave out the ‘offs’ and follow the course carefully, progress is virtually guaranteed.
As Adrian said, slowly and cleanly is key, then repeat endlessly in 1-minute chunks.
You’ll be surprised at your progress. And remember, everyone learns at different rates 
Hi Aarin…my advice is to slow down and focus on the different skills required to play guitar separately…do your chord changes nice and slow with just one downstrum, introduce a metronome as soon as you are ready. There’s no reason, if you persist, why you won’t be able to speed up the changes.
It’s very good that you try to keep the hand moving…again…work on it separately, for example on muted strings ( an exercise that I often do is to strum along with the original recordings on muted strings, it’s so much fun and much beneficial too).
You will add the the chords when ready, and a very good idea would be to use only downstrums and not strumming patterns to begin with.
The reason for working on our skills separately is simple (Justin mentions it in a lesson but I can’t remember which): our brain can focus on just one thing per time: if you’re not 100% confident with your chord changes you can’t share the focus on trying to keep the hand moving, one of the two needs to be automated.
Hope this helps…keep going and be aware which skill you’re working on in each exercise you do, with consistency you’ll be fine.
Loads of great advice already given in each and every comment above me. Only one little thing I’d like to throw in. As you mentioned:
To fix this, in addition to the One-Minute-Changes, there’s another thing called Chord perfect practice, that you can do in addition to the OMC, to get the chords also to ring out clearly. I can’t seem to find this lesson right now, but here, you take it really slow and place each finger softly on the string. Once all are placed and you think it’s right, press down and strum. In case you have buzz or mute, isolate the finger causing the problem and adjust until it works fine. Take your hand off, start again.
Later on, there will be a combination of both exercises as given here:
https://www.justinguitar.com/guitar-lessons/perfect-fast-changes-b2-802
Also, try to take off the pressure: Tension might also be involved causing problems in the long run. It’s a marathon… 
Most of all: Have fun playing guitar.
I’m also having some trouble with chords. Chord perfect works pretty well, but I find I have to hold my left hand in a very unnatural position, which takes up unnecessary time.
I always practice sitting on a couch, so I don’t think my sitting position helps much.
So I’m going to look for a height-adjustable stool and perhaps a footrest.