This is not black and white to me. I hear you about the negatives but you also have the opportunity to ride a lot of horses each day, an opportunity others would kill for. That said, are you reaching your goals? Is there a way you could pay for a lease or half lease so you can work towards what you want to accomplish without the rug being pulled out from under your feet? I was a working student for many years so I understand that many in those positions do not have the ability to pay as much as a regular client in many instances. But maybe you could pay a lease fee for just the show. That way the trainer wouldn’t hand the free ride over to another client?
This is a job and most jobs aren’t always peachy every day. That said, you should be looking out for your own goals. Be careful about sneaking around. It will get back to her eventually and if you need a reference and this person would be the major ref then don’t burn a bridge. I had someone do that to me after working for them for a year and once I left, even giving notice, she talked crap. Which really hurt as I did anything and everything for her.