I had an OTTB with the same issue - as I believe that's how they are trained. We went back to lungeing and re-learning vocal commands for walk, trot, and canter. Then when I rode, I would repeat those commands and use your seat and legs to reinforce. It's going to take time and patience (of course, doesn't everything!!). As jazzyrider said, riding with steady hands that are very gentle is a good start. Really concentrate on using your seat and legs for cues.
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My name is Lisa, and I am owned by Lucius and Sawyer.