what i see is you lack a true solid foundation in your lower leg that is causing your leg to slide back and your upper body come forward. you are also jumping ahead and your horse seems to be lurching rather than rocking back and pushing over the jump.
first, i'd recommend some flat work w/o stirrups as well as in all two-point to strengthen your lower leg, deepen your seat, and help you be more secure in the tack. next, i'd work over small gymnastics no more than 2' high and at least 3 - 4 in a row set about 1 - 2 strides apart. if you don't have experience setting gymnastics, please get the help of a trainer! in doing the gymnastics, ride in two point the entire time and close your eyes (yes, really!) this way you can get the feel of moving with the horse without anticipating the jump and forcing your upper body up and out of the saddle too soon which is what you are doing now. the more you strengthen your lower leg, and wait for the jump to come to you, the better your position will be and the less you'll find your heel riding up, leg slipping back, and pinching with your knee. close your fingers on the reins and lower yourself into the tack more and let the horse jump for you.
in addition to helping your position, it will also help with your horse's drapey front end and sloppy take off spots, forcing him to also relax and balance and engage his hind end more - yes, even over jumps as small as 2'.