The last time I looked for a horse (and ended up with a Morgan mare), somebody was giving an ASB away. A friend got wind of it and told me to pass, that the horse was so nervous that she would constantly stall walk. Nobody knew why, and she kept losing weight because of it, no matter what or how much they fed her. I don’t mean to discourage you. On the contrary, I want to reassure you that it’s not uncommon! How often do you ride her?