It could be a medical issue, I suppose, but more than likely, that isnt the problem. Some horses are just that way, Ive fooled with many horses that have been like that. You could go the route of getting a professinal trainer and spending more on her than 20 good horses would cost. I have very rarely had a horse that couldnt be broke or trained, but they do exist. Its just a flaw. Ship her on and find one you enjoy.