First of all I think you should be proud of yourself for making an effort and at least understanding what the goal should be. That's about 95% of the battle right there!
Were these pictures posted in chronological order? Because there's a bit of progress through them! I think the first 2 pictures really show why you've gotten into the habit of not releasing. It all started with your leg. Or lack thereof. Especially in the first pic, you can really see how you were completely on your knee, which throws your lower leg back, losing your foundation completely. So to compensate your upper body has thrown forward on your horse's neck and you are supporting yourself on your hands. With this picture alone George Morris would have said that you should go back to cross bars and reeducate your leg over the fences! The second one shows the same habit, but done in a slightly less dramatic fashion. Still on your knee, supporting yourself with your arms. But looking more stable. still, difficult to release when you're on your hands like that! The progression to the 3rd picture is HUGE! Your leg is significantly better, you're much more over the center of your saddle, and you're able to release much better. Wow, huge improvement!
My guess is that because you started off with such an unstable leg it became habit for you to rely on your hands for balance, and even as your leg/support has improved considerably your hands are still in the habit of locking in place. Watching your video I also see a very stiff arm in between fences, almost like you're trying to clamp them in place. Think of your joints being smooth and following. Instead of holding your horse your'e going to follow his mouth. One trainer I worked with called it "oily joints". I think that sounds kind of gross
but it makes sense. As fluid as you can. Then think of the jump as just another part of the canter, your hands are simply following your horse's mouth in a fluid motion. If your release is that much of a bad habit, you may have to really exaggerate the release for a while to get used to it.
One of the absolute best ways to work on position over fences is through a series of gymnastics. Over and over and over. Go through a million times until your leg learns to stay perfectly still. Only when you have a correct and stable leg can you really fix everything else. Then work on keeping your upper body still as your arms stay fluid. Have you ever jumped without reins? They don't have to be big fences, but tie your reins in a knot, hold your arms out to the side and go through the gymnastics working on letting your body depend soley on your legs. That's a really great exercise!
I think you should be encouraged though. I see quite a bit of improvement!