I completely agree with Stockshowkid97’s post, but I will add one more bit to it. How does your mare do at an extended trot? I would lunge her for 20 minutes before you get on, gear and all, and of course canter her for a lot of the lesson. When you ride, I would keep her as calm as can be, like Stockshowkid suggested, and speed her up a little at a time. I’m quite curious to how she reacts to an extended trot. Your mare most likely soured and suffered horrible anxiety on the track, which what is causing problems now. I’ve seen barrel horses with the same problem. It’s going to be a while before you can gain her confidence to run with you again. Keep patient and take your time, and you’ll have a wonderful horse eventually.