In addition to smrobs comments, I noticed that your lower leg frequently is too far back, rather than firmly underneath you. Sinking a little more weight into your heel and pushing your lower leg forward will help, as will trotting in two point or half seat allowing all your weight to sink down into your heel. If you're riding dressage, it's not necessary or desirable to have your heel sunk as deeply as a hunter or jumper rider, but you do need to have it flexed enough to secure your leg. If you glance down while riding, your stirrup leather should hang perpendicular to the ground. If it's at a backwards angle, push your leg forwad until it's perpendicular.