Your equitation is not bad because of your body, it is because of how you use it. You need someone to be there to correct your position, and no one can really help without being able to see you. If you can, get someone to do a video of you while you are riding, analyze it, and then work on whatever you need. Riding with a bareback pad can be very helpful, because if you aren’t using your legs, seat and body correctly you will not be able to stay on the horse. when I was young, we had a chute with 3 or 4 jumps in it, and were required to go through with no reins and no stirrups – really helped with balance, and made us be correct without interfering with our horses’ natural movements. Nowadays, I see a lot of emphasis on “hit and kick”, neither or which is ever (my opinion) proper.
It is never the horse's fault