I suspect the saddle is part of the issue; it looks like your seat is being pushed to the back of the saddle and your leg pushed forward. I think the saddle is a poor fit for you, and that that the balance point of the saddle is too far back as it's sitting on this particular horse.
If I am purely looking at your position, the first photo is the best. In the others I can see you fighting to keep your leg under you.
[quote] I'm used to riding hunt seat, with my leg forward and my heels way down. [/quote ]
This statement confuses me. In hunt seat riding, your stirrup leather should still be perpendicular, and the alignment of your shoulder, hip and heel is the same. The difference in a dressage seat is mostly the length of stirrup and more open knee and hip angles; and spending more time riding in a full seat.
As far as heels, some hunt seat riders tend to exaggerate heels down, to the point that they force them or brace them, this is not correct and robs you of flexibility and shock absorbtion. It's common for a dressage instructor to advise you to release some of the weight in you heel so the joint stays soft and unbraced, but your heel is still down - just not forced down.
Is it possible that bracing your heel in the stirrup helps push your leg forward? When you release some pressure in the stirrup does your leg come back underneath you? In the video, I get a sense of locked joints and stiffness, and that you're posting up from your knee rather than opening and closing your hip.
Last edited by maura; 04-17-2011 at 04:32 AM.