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.
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