For jumping ahead, I always learned to jump by staying in a 2 point approaching all my jumps. I used to jump ahead much more seriously than you do. Just lock your heels down, get in your two-point way ahead of the jump, prep your body find your perfect two point, and just practice not moving. The horse doesn't need your help to jump, just let him/her come up under you. The only way you should be moving when your approaching a jump is just moving your hands forward to allow a release.
Even then, I'd work on your position first and worry about releasing once you have a solid position over the jumps. As long as the horse your riding is forgiving and honest to the jump, I'd just keep a pretty loose rein and practice your position over low jumps, and not worry about releasing. Once you learn not to anticipate the jump and your muscles strengthen, the release should come naturally.
You anticipate the jump, and try to jump it for the horse, which is why you jump ahead. You just have to learn to let the horse do the work and keep your position steady. I've found for myself, that my leg comes back more the weaker my leg muscles are. As much of a pain it is, riding as long as possible in a two point on the flat is going to help you 10 fold over fences. You'll strengthen your back and leg muscles, and even though you'll want to die during it, you'll feel the difference over jumps.
The thing that helps me the most is finding my two point a couple strides out, before the jump, and just concentrate on not moving a muscle through the jump or line. You can usually (or at least I can) feel your faults as soon they happen. I can always feel when my leg slides or my heel comes up, I can definitely feel when I jump ahead. It's all just building muscle and learning not to anticipate.
I agree too, that shortening your stirrups will help. I consider myself a dressage rider by nature and I love myself a looong stirrup. For hunter flat I'd shorten it at least a hole from your dressage saddle, and over fences another hole. It depends on the horse, too though. Fatter/wider horses and ponies will make up for a hole in your stirrups.. I'll have my stirrups adjusted for jumping on a slender horse, then put my saddle on a wider large pony and find myself shortening it a hole or two. That may just be me though.. :P