Cute horsey =) It looks to me like your horse is tugging on you a bit(is this the one that gets heavy???). Lots of times when people have a horse that likes to hang on them , they step up on their toes a bit to brace themselves. Try to concentrate on keeping your leg lengthened down and wrapped around your horse's barrel. Keep a good contact with your thigh and your calf. You want your knee on the saddle, but you don't want to pinch with your knee. Try to always keep some contact with your thigh no matter what you are working on with your horse, and in the air over the jump try to squeeze with your thighs to keep you tighter. I hope this helps and is along the lines of what you are looking for....if not I am sorry for rambling!!