It’s very courageous of you to reach out for help, and I’d say wise as it would be beyond sad if a bad accident happened to you. I never write in forums as I’m overwhelmed with workload but I felt I should take time to suggest things to you in addition to all the good comments you already got.
Just to introduce myself, I’ve been riding since I’m 4 and it’s been 41 years now, I’m french and have been introduced to equine osteopathy, and lots of other healing techniques, as I’m a healer (but for humans).
I don’t know what your level of experience is, but it seems that your mare is a serious challenge. This challenge can take you to expand your world, your knowledge and become a more accomplished rider. Just be aware that it can take a lot of time to find solutions to challenges and you need to know if you ready to invest that time and energy. As others comments have pointed, nothing is worth ending up in a wheelchair because you didn’t listen to warnings. Your mare’s behavior is a serious warning that something needs to be done.
Your mare’s problem is behavioral, but it might have so many origins that finding the right one is crucial.
It might be just genetic, as AA are hot blooded and take experienced riders, or riders who have a good trainer to help them
It might be nutritional or hormonal
It might be “psychological” in the sense that she decided that she’s the alpha and not you and she can do whatever she wants (you can watch a 1hr video with Pat Parelli demonstrating just that on youtube)
It might be that she’s in pain… If one or several vertebras in her back are “restricted” or inflamed then when you sit in a certain way on her back or she makes a certain move, the pain goes off and off she goes. I see so many horses blocked in their backs, or their shoulders, or their necks or their hips and it baffles me that no one else sees it. If you post a video of you lungeing her on both sides, walk, trot and canter and you send me the link, I will take the time to watch enough to be able to tell you if I see something or not. Alternatively, you can look at Jean-Luc Cornille’s website and read his articles to understand better what I’m talking about. Or you might want to have her checked by a really good chiropractor. The ones I’ve seen didn’t do anything good to my horse, I had to do the work myself. But I’m sure there are good ones around, you just need to find them, just like trainers. As you can understand having your saddle fit her back well isn’t going to fix the underlying osteopathic problem if problem there is.
Once again, I find it extremely courageous of you to be willing to work with this difficult mare. Don’t pay attention to people who don’t understand why you can just gallop ahead or berate you because you can’t fix her in 10 seconds. It’s going to take you time, practice and patience to resolve this challenge and it’s normal. Trust yourself, listen to your intuition, find the right help to keep you and her safe.