OP what you describe is awful horsemanship from the ‘trainer’, you are not mistaken. And you are in a tough spot – wanting to continue your equestrian pursuits with limited options.
Let me preface the remainder of this post by saying that what I’m offering is quite possibly a phenomenally bad idea, that will likely lead down a frustrating road, if it leads anywhere at all. Having said that…
Reading your story, putting myself in your shoes, and only having your words and my imagination to go on, I personally would consider going back, but not exactly for lessons.
If it were me – and if I felt that I could be successful with this mare if given half a chance – I would consider going back and requesting another lesson on the mare to try again. I would tell the trainer that I’d been thinking about this mare, maybe make up a story that I once rode a horse like this and ‘with patience and time’ was able to make headway. I’d pretty much say anything to buy myself an opportunity to ask the trainer to give me the first 30 minutes of the lesson to ‘try things my way’ and see if anything came of it. Of course still paying her for her time, etc. I wouldn’t expect to pay a dime less.
I’d be friendly with the trainer, chat amicably, and cross my fingers and toes that in 30 minutes of quiet, gentle riding, I could coax the mare to loosen her stiffness a bit. Try a few options to encourage a shoulder in, some spirals, etc. ANYTHING convincing of progress.
My goal would be to demonstrate a better way of working with the mare.
Then, if I were successful, I would hope that the trainer is a big enough person to accept an outsiders opinion. If all of those stars and moons aligned… and that’s a big ask of the universe… I would see if this opens the door to riding her school horses regularly, for an hour, the first 30 minutes being mine alone with the horse, the second 30 minutes instructions from the trainer.
If the trainer had any scruples, she’d connect the dots, do the math, and see that she was getting way more for her money than just a few dollars in her pocket and would encourage my returning to tune up her horses. In return, I’d be getting saddle time, and a chance to foster a relationship that could extend to a future kinder, gentler hand with the horses in her charge.
If, in time, I proved to be of value to the trainer, it could work out to a light-fee, or entirely free, opportunity to come out and ride her schoolies to keep them honest. That would possibly free up funds to venture out once or twice a month for quality instruction somewhere far away.
Sounds all very Disney movie-like I’m sure, and perhaps that’s just how my brain is wired, but I personally am Pollyannaish enough and stubborn enough to try to pull a stunt like this off.
It all could go very very badly though, so again, let me be the first to suggest this is quite possibly, almost positively, awful advice.
Good luck!