She needs to start on the ground. Start over . Teach Stop. Walk. Whoa. I have some TB's one a mare, not good at the track, but she still needed whoa taught. They gelding who did decent and was raced until an old age for race horses 13 or so, you could not make run he would buck or rear when asked, but he also needed taught whoa. I was not the original owner and these horses had gone through many diff people before me. They came with behaviour issues and we just started in the arena to gain some respect and reteach the basic, stop , walk, turn left turn right and back up ... Have fun !!