Last year I bought an 18yr old mare that doesn't seem to know how to canter. She has a beautifully smooth trot or jig. But if I ask her for more speed, instead of breaking into a canter, she speeds up her trot, at which point her trot becomes very rough. She seems reluctant to break out of this rough trot, and when she finally does, she breaks into an all out gallop. What is even more disheartening is that she does the same when she's out on the field w/ the other horses. The other horses will be cantering across the fields, & she'd be trying to keep up w/ her rough trot, until she breaks into an all out gallop. Anyone has any suggestions?