You are probably gripping with your knees over fences. Your knee is acting as a pivot, sending your lower leg backwards and your body forwards. You need to learn to wrap your entire lower leg around your horse and sink your weight into your heels. Once you have a secure base of support you need to break the habit of throwing your upper body over a fence. Jumping without hands and eventually without stirrups will help a lot.
One thing you can do now to help is to trot in two point over trot poles with no hands. You can either have your instructor lunge you on your horse or you can tie your reins in a knot and place the poles along the wall, as you come around the corner drop your reins and place your arms out to the side like an airplane. Focus on sinking your weight deep into your heels and keeping your shoulders back.
To help my legs and balence, I just tried to stay in two point for every gait. First walk, then trot, then canter. Without holding on. It was kind of hard at first! But it gets easier with more practice.
Next time you jump, try to feel if you are gripping with your knees. If so, you need to work on gripping with your calves. Try to hold a 2 point position without your legs slipping back. Once you can do this once around the arena, try twice, then three, then four, then more. Then you can start practicing again over small jumps. If you feel yourself pinching with your knees, go back to 2 point until you feel ready to try again.