Okay, then you need to fix how you hold the guitar. It should not slide out.
I had a skim through your learning log and found the video of you practicing HOTR a month ago to have a look at how you’re holding it. And strumming.
I want you to compare it to this video of me holding my electric seated playing Sunshine of Your Love. I use a video of me as I know how I hold it…
Of course we’ll have to imagine a bit about what’s happening out of frame, but we can make assumptions based on what we see
Things to check:
Seat should be low enough so that leg is roughly flat - it’s OK if it goes down a bit.
Sitting up straight. slightly forward is OK, slightly back is not OK.
On the vertical plane, the guitar should be roughly straight up and down. It’s OK if there is a slight tilt backwards to it.
It looks like the guitar is right in front of you. I assume your legs might be too, based on where your guitar is. Your legs should be pointed in the direction of the headstock, so the guitar is against the side of your stomach, not the front. This would mean your torso would be ever-so-slightly twisted towards the guitar, with the legs the other way.
This has the effect of bringing the body of the guitar further across the front of your body. Your strumming arm is then not as far to the side, it can be more relaxed. Compare your videos and mine - my guitar is more across the front of my body, yours is sticking out the side of your strumming arm. It’s subtle, but it makes a big difference.
Once the guitar is in the right place, you should be able to anchor it in place with the upper part of your forearm. Look at my video and see the upper part of my forearm. That’s holding the guitar in place. I’m not pressing it there hard, it’s relaxed, but it is there. You can see it stays anchored when strumming as well - I move a bit but can both strum and pick from that position.
It’s really hard to get this right unless the guitar is in the right place. If that guitar is not in the right spot, your right arm will be too far back, and you’ll get a sore shoulder quickly. If you get it right, it’s comfortable, relaxing - you could play all day. It should stay gently anchored in place.
Hope that helps - kind of a challenge to get this stuff in words. If you get this right it will make playing way easier as the guitar won’t move around while you’re trying to use it.