You're rounding your back quite a bit and collapsing in the core. Additionally, I see a lot of weakness in your lower leg. The insecurity there is causing you to fall forward in some instances and other times you're thumping down on to his back. Often you end up pinching with your knee and pivoting forward, which is another product of the insecurity/weakness of your lower leg. Try shortening your stirrups significantly. I've found that that helps a lot.
Really make sure that you're sinking your weight down the back of your heel. Everything should come from that, not your knee.
Doing a lot of work in two-point on the flat would be a really good move at this point. One of my trainers once said that her trainer had her ride her entire intermediate dressage test in two-point... Therefore we have no right to complain. ;)
Also, GET UP off of your horse's back! Shorten your stirrups (yes you can wrap around the iron twice), practice a lot of two-point, but you NEED to stop sitting down. There's a time and a place for sitting, but at this point you need to be up. Again flat work in two point (insane amounts of it) does wonders.
Make sure that you're up off your horse's back in half seat/two point and then two strides before the fence, sink just a bit. Don't sit down, but sink. As for releases, when you feel your horse rocking back to jump, you begin to release. Your release isn't bad, though, so don't worry too much about that at the moment.
As for the way you handle the horse... You're not setting him up well all of the time. You're burying him into the fence a lot, and not helping him get his distance at all. He's obviously a really sweet, fantastic guy. :) Work on counting. Count out loud! Don't be embarrassed if you count the wrong stride: that's how you learn to get it right! :)
Your horse looked pretty exhausted in some of these clips, so make sure you're watching him, and not pushing him too hard or making it unpleasant for him. He's a VERY nice boy, so be careful with him!