There are many ways to improve your jumping position.
First of all you need to work on dressage without stirrup to fix your legs and chest, work a lot on descending transitions.
The most efficient is to work on combinaisons on a ligne, it will help you follow the jump and work on your reflexes and suppleness.
Then, you can jump a ligne without stirrups (it will help fixing your legs) or without holding the reins (it will help fixing your chest and finding your balance) or even while closing your eyes (it will help "feeling" the jump).
If you feel like you don't follow the jump, it means you don't see correctly your strides coming which is very important to know when your horse starts to jump and when to follow him. You can work on 2 ground poles separated from 5 strides and then try to put one away (4strides, push more your horse), or one more (6strides, hold him more).