Can you not refuse to do as she asks? If you can't leave her, you can atleast stand up to her and potentially keep your horse from being ruined (if he isn't already.) She's your trainer, yes, but she does not own your horse and thus cannot force her methods on you. If she threatens to quit, tell her that she's free to do so. Once September rolls around (luckily that isn't too far away) do as everyone else says. RUN. Find someone who's willing to teach
you and your horse instead of using pain to force things on you. Any real trainer knows that headset doesn't come from the reins, but from collecting the body. Find someone who you can watch ride a few times, try sitting in on a few of their lessons with other people (if they'll allow you to) and who seems to genuinely want to help you improve. Neither you or your horse deserve that kind of treatment. Don't put up with it.