Since you don't know much about the horse's background, and since it happens with your trainer as well, I'll tell this story. FWIW:
When I bought Mia, I was told she was "Perfect for a beginner!" I rode Mia in a sidepull halter for 2+ years. She initially didn't want to go the way I asked her to, but it was hit & miss. Eventually she got pretty consistent about turning left with a left opening rein, which is what you have to do with a sidepull - pull the head to that side, not back on the rein.
However, Mia became more nervous as I asked more of her, and we eventually had a 2 hour blowout where I couldn't get her to stop and I took off a lot of hair with the gentle sidepull halter.
She then sat unridden for 8 months while I took lessons and rode Trooper.
The lady I hired to work with her was obviously puzzled by Mia. After 4 sessions, she told me some horses just had problems that couldn't always be trained out of them, but she didn't want to give up yet.
After the 5th session, she told me she had concluded that Mia had never been broke to ride. She said there were just too many things that were fundamental to even a green broke horse, and Mia didn't have them. The good news, she said, was that instead of having a horse who couldn't be trained, I might have an uncommonly good-hearted horse who needed breaking. So she started Mia at the very beginning, assuming she knew nothing about being ridden - in spite of my having ridden her for over 2 years.
Did it solve all of Mia's problems? No, but it laid a solid foundation for her to understand what I was asking. With all 4 horses that I've owned for a year or more, I've noticed that what I was sold wasn't what arrived at my corral. If I bought another horse now, I'd probably automatically have a trainer put at least a couple of weeks into working with the horse to assess what it knew and what it needed to know, and to make sure it had the fundamentals. I'm sure I could tell a trainer what I expected (which isn't much), and have the trainer tell me in a week of work if the horse had holes that would cause me a problem. At $50/session for 5 sessions, I would consider that $250 well invested.
In the 5+ years I've owned Mia, the 3 best decisions I've made with her were:
1 - Stop riding her and take some lessons.
2 - Have a trainer start her over from the beginning.
3 - Switch her to a curb bit even though she didn't neck rein and wasn't a 'finished horse'.
In fact, that is probably how I would rank order my good decisions about Mia, too!
Good luck! It may not apply to your horse, but it is something to toss in the back of your noggin! And note, it is a bit surprising, but this picture:
Was taken at least a year before this picture