I was at a clinic and since the advice for you seems to be that you need to keep your heels down more, let me throw in my two cents. I was told that my weight was pushing against my stirrups, rather than sinking below them. It helps me to think about wrapping the inside of my heels around the horse, while turning my toe slightly out. To strengthen your leg and your position I'll second a lot of two-point work. (If you want to strengthen your core, which is a big thing for dressage, then do no stirrups work) Good luck, I hope that helps!