I assume you have a trainer? Does she know some of the really evil jumping exercises? Like . . . making you jump with your eyes closed? That will fix that jumping ahead problem. As you approach the jump, you have to close your eyes. You do that a couple times, and if all goes well, next time you close your eyes and hold one arm out to the side. Then both arms out to the side, eyes still closed. Then the hardest one, both arms reaching to the sky (eyes still closed).
I had an evil, evil jumping instructor. She couldn't make me a great rider, but she sure improved my position.