I had the same problem with a mare that I uses to have as a kid. Everyone thought I was crazy, but I was scared to death of having to try and canter on her. Came to find out that the people who had her before me ran her as a barrel horse so the harder I pulled back, I swear the faster she would go. We finally switched her to a bossel which worked better, but the trust between us was already gone. I stopped riding after that, the fear was too much.
I started back again recently with my daughter. And I'm sorry everyday that I didn't stick with it. I suggest that you find someone who is experienced, like an instructor, and get their help with the problem. That may have been how she was trained, and she just needs help being retrained.
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