If you are trying to focus on your position do it on trained horse, that way you only have to worry about yourself. To prevent your leg from slipping back, try riding without stirrups, keeping your toe pointed to the outer branch of the stirrup iron, and avoid using your legs to push yourself into two point (your leg position should not move, you should only bend and allow yourself to be pused out of the saddle with the horse's movement, not push yourself out of the saddle). Another reason your leg may be slipping back is your stirrup length (which for jumping should be about two holes higher than your ankles) or the saddle is the wrong side. Good luck in the spring! I hope this helps
When you're approaching a jump go into two point and then just stay like that don't move. Keep weight in your heels your horse will come up to you. Bareback and stirrup less work too.
Try jumping w no stirrups it help u keep your balance and structure xo
Put your heels down and keep them down then shifting you weight into the balls of you feet
Drop your stirrups - this will help you wrap your leg around the horse and keep it still. You can also try two point in the trot and canter and go over course.