I wish my leg slipped back that much!! When my leg starts slipping back in terms on centimeters instead of feet, I'll be in good shape. You look fantastic. You look a little big on him, but nothing to where it looks like it's time get a bigger horse. I can't tell from the picture angle, but since your stirrup length looks good, maybe your pinching with your knee just a little instead of distributing your leg evenly along the girth which is causing you to jump ahead a tad?