What helped me a lot was jumping without the reins, just focusing on my position! I did that a lot, small jumps and some jumping gymnastics, you'll start to feel your balance and where you have to have your body so you don't go too far up on the horse's neck!
Hope it helps
First you need to know that if you go too far up on the horse's neck, you will disturb his balance (he will have mire weight on his shoulder and can do more faults such as pole down, or even he can refuse to jump).
You need to really wait for the jump and don't jump before him.
To do so, you need to fix well your leg (it will give you more balance), put your weight on your heels own, do a lot of jumps without stirrups to fix it.
Then, to make sure you wait and don't "fall" on your horse's neck, you can try to jump without your hands or by closing your eyes, it will help you wait for the jump and have a better position.
Try sinking deeper into your heels, grab a chunk of mane and almost think like you're going to be left behind. It might help to practice this through a grid and then over single fences. Hope this helps!