I become a stress monster at the canter transition as well. I have ridden a couple of horses that have caused me major frustration with correct leads, bucking, etc when I ask for the canter. As such, I delay my transition and wiggle my position all around because I'm so nervous to ask. I am taking a "yoga for the dressage rider class" taught by a certified yoga intructor/dressage trainer. Sounds nuts, but I find him saying a lot of the same things my trainer says when I ride. Being aware of your body, getting everything relaxed, staying "emotionless" and calm. I will let you know how it goes-lol- I've only been to one class, but maybe yoga can help!