Make sure you stay with the horse. Your leg wont fall back if your timing is right and you keep the weight through you heel.
tie your stirrups to the girth with bailing twine, it'll help teach the muscles to stay there
Freejump stirrup leathers
Push your leg forward when going into two point , also make sure your not staying in your two point too long!
No stirrup work helps sooo SOO so much!! (Even tho it's painful haha!) but also try going with the horse over the jump and pushing your heel down. When I got told at first to push my heel down & around the horse I could never do it but if you keep trying it helps!! Also do not pinch with your knee!!
But that might only be for flatwork idk
I know there's a trick where you tie binder twine (it'll snap if needed) so your legs can't go back as far, even though I've never done it myself
Staying balanced over the jump is important and if you can, you could try visualising your leg sticking to the girth point on the horse.
Push your legs firer and heels down while going over the jump
Think about pushing your heels down just as the horse takes off. Top international showjumper said that to me.
You could try to do no stirrup work. I have the same problem, and my trainer says that it's all about your balance. Make sure you stay centered over the jump and your leg shouldn't slide back too far.