Try to only lean forward over the jump push your tush back and bend your allows down to the horses shoulder and have your hands lined with your chin
Try bending at the hips rather then your back; have the same back position over fences as you do in the sitting trot, only difference is bending your hips! Hope this helped!
I won't go on and on because I have a lot of say about position of fences lol but mainly I would say work a lot on that core. Do workouts outside of finding to build up a strong core to help balance yourself and keep your body strong over the fence. Also one of the big things that helps me is watching other people. Look at their techniques, where they hold their hands, how their shoulders are held, and how they are perched over the saddle. It will take a while really get your eq where you want. Take it from me I am constantly working on my equation over fences, changing it and watching how other people are riding to see what works best for me! Hope this helps!
One major thing that improved my positioning over fences ir keeping your heels down😊
Let your elbows be really elastic. If you're having problems following your horse's motion, as some horses have really eccentric jumps, don't be afraid to grab mane. It will teach you where your hands should stay while jumping. Also, remind yourself not to be in front of your horse's motion/on the forehand. If when your horse lands, you feel like you're falling forward, that's probably why. Try pushing your bum backwards and your arms forwards. Heels down and leg on will also keep your balance, but you probably know at least most of this already. Sorry if I'm sounding a bit repetitive. Your jumping position is probably a lot better than you think it is, and better position comes with practice. I'm still working on it myself. I hope I could help!