I agree with the "standing in the stirrups" comment. You need to relax your lags and allow your seat to move further back over your saddle. You are jumping ahead of your horse too much. Relax those knees!! Allow them to absorb the movement and let your seat move back over the middle of the saddle to maintain your flow with the horse's movement.
I'll use some of my photos to illustrate. Click on the photos for a bigger view.
See how much further my seat is over the saddle?
Even though my ankles are not as flexible as I would like (I've broken both of them and they do not flex too well), I am in good balance over the horse. If I were in this same position with my feet on the ground, I would still be balanced. I would neither fall forward or backwards. That is an important part of flowing with the horse.
Todays reliance on the hated crest release has taught rider to perch on their hands for balance and never learn to be properly balanced on the horse, IMO.
Here I am over a much bigger jump. It looks like I am jumping ahead of the horse, but the question is...would I be balanced if my feet were on the ground? Would I topple forward or backward? If I was too much further back in the saddle, at this MOMENT, I would be behind the movement and interfering with the horse's flow over the jump.
So, different jumps demand different positions. Take photos of yourself, and ask....am I balanced? Work on freeing yourself from balancing on your hands and strengthening your lower leg position, and you will improve rapidly.