Good suggestions from @Another-Bay and @Scribbler.
I’d add to check your stirrup length. If your stirrups are too long or two short, that may prevent you from achieving a proper position. effect your seat and your balance too.
Jumping takes a combination of strength and balance. For strength, if you aren’t doing so already, ride without stirrups for part of each ride. Do walk, trot and canter without stirrups. When you can post the trot and stay in two point without your stirrups, you will be able to hold your position much better over fences. If its too intimidating at first (which it can be, especially on a green horse), have someone lunge you to do the no stirrup work until you are comfortable with it.
To work on balance, riding with no reins is a sure fire way to increase your ability to balance yourself without relying of the reins to correct yourself. Do this with someone lunging you - just tie the reins in a knot, or remove them from the bridle - work at walk and trot first. It will feel odd without anything in your hands at first… take your arms and do the “airplane” (arms out to your sides), arms on thighs, arms on hips, arms straight out in front of you. Once you’ve mastered posting trot, move on to canter.
While this may be a really “old school” method, I promise it can work wonders if you stick to it! 