Shorten your stirrups a tad, and get off of the horses back! All your body is laying on him! Your whole stomach, and everything is lying on the horse, causing it to be 10 times harder on the horse to jump.
Lets start at your seat! Your stirrups need to come up at least two holes, possibly more. This will put your thigh ahead of you more and so keep your position more secure. Mine go up 5 holes from flat to jumping.
Then work on the ground to develop your balance for jumping.
Fold your body from the hips until your shoulders are in a verticle line with your knees and toes. Your hands will now need to go ahead of you so you'll need to shorten your reins. I teach the automatic release from day one - I honestly cannot abide the Show Hunter positions! Your hands then need to go either side of the horses neck so that you have a straight line elbow hand horses mouth. If your body is correct then you will have no problem doing this. Look ahead of you so that you know where you are going - to the next jump not the ground.
Youe seat will need to move back a little and will lighten itself in the saddle - you don't deliberately lift yourself off the saddle. Once you acn maintain this position well in walk trot and canter then add a small fence. Being able to sit still and let the horse do the jumping is quite hard for some people - they want to throw themselves over the jump before the horse.
In the air, keep your body straight over your horse so that he is able to jump evenly and cleanly, tipping to one side will cause the horse to drop or dangle a leg.