My foot frequently slips from the stirrups when I kick the horse, especially while I'm kicking with one leg to get the horse's hind end to turn. I'm not sure what I should do. I'm letting my body weight sink down into my heels, yet I'm still having that issue. My stirrups are the correct length as well. I'm still having trouble kicking the horse while posting, I miss a beat and am on the wrong diagonal, or I slip and lose my balance completely. Does anybody have any tips for these?