I always felt like understanding of why your leg and seat have to be the way they are really helped me peice together my position (although im not yet close to amaaazing) bc its not about looking good so much as getting a feel for the purpose of your seat bc different disciplines have different priorities (eventers dont release as much as hunters etc)
Go into the 2 point position (on the horse) and stay there till you find your balance. (don't lean on the horse with your hands). Sit back down but don't move your legs, and that is the position your legs should be in.
As a BHS Instructor I have a million types of exercises for posture and technique improvement! It does depend on the specific issues you are dealing with. A paper plates held under your thumbs keeps your hands still when riding. Posting in the trot with only one stirrup (you alternate the one used) improves balance and strengthens core/thigh. Tying the stirrup to the girth creates stability in the lower leg and correct muscle memory when jumping. Ultimately a great trainer will be able to help you with your problems!!