I had SUCH a hard time learning to post (you know, that up down, up down thing) I would bounce all over the place, get cramps in my side, out of breath, etc...then slowly I started to get it. It just takes time, my guess is because you are so athletic you thought it would be no big deal to learn riding, but with any other sport, you use different muscles. I was a runner in school, in cross country and competed in running races over distances...pretty good too if I do say so myself. But when it came to riding I was so 'out of shape'. With english riding you use those inner thigh muscles you don't really use in any other activity. Try doing some exercises to work those muscles out, that will help you.