I had the same problem, my coach in France adviced me to have a two point position “In two-point, you stay off the horse's back by balancing your weight in your feet. Your upper body is bent 20 degrees forward from the hip. Your seat stays over the deepest part of the saddle. Your heels are stretched lower than your toe. You can also think "raise my toe". You can also imagine you have pins on the saddle so you do not sit on it while jumping a course.
This takes some practice and strength. Take a look at YouTube for examples. “
It really helped me as well as doing some wobble board squats to improve my balance.