Lots of no sturuip work!!
No stirrup work and try jumping with slightly shorter stirrups :)
A lot of bareback and my trainer tells me to "kick the ball" so pretend that you are kicking a ball as you go over the jump. It has helped me a lot!
Try doing some work bareback at the walk, trot, canter and the jump. You will feel unbalanced if your leg is back. While flatting (bareback) practice on opening up your thigh and making your legs longer. Do small jumps and continue to practice that. While in the saddle when you're approaching the jump just go into 2 point and don't move. Don't move your legs or your body really the horse will fold you up. Or you can do as Sophie said. Tie your stirrups to your girth.
Make sure your heals are down xx
I would keep your heel down and make sure you iron is the correct length then if that is all good then the your iron to your girth with some twine or a spur strap. I did it and it worked great
I have that same problem. what my coach did was she tied my stirrup to the girth so I couldn't move it back
Do a lot of no stirrup.
And then try doing some little jumps with no stirrups/or bareback, it'll teach you to keep your leg from sliding because you'll have to really have your leg on to make sure you don't fall off or anything