The more I think about this the more I think pain.
This horse is 4. If she came off the track at 2 or 3 what kind of letdown or re-schooling did she really have? Did she go to a clueless newbie that got promptly tossed? How long did she stay there?
I would assume this horse has no significant re-schooling and has unaddressed pain issues which could be just tensions and ulcers or something more serious.
I agree with the advice about being really choosy in future with project horses. As an analogy, the top colleges in the USA can boast of their students success later in life because they are so careful to admit only students who are obviously going to be successful. It’s a clever and self perpetuating strategy.
Now some trainers carve out a niche for themselves as fixing problem horses, but even they tend to take on clients horses, not buy problem horses themselves. “Owner selling cheap because they got tossed” is a huge red flag. Most owners who come off “innocently” because of a minor stupid spook while they were not paying attention or get jumped out of the tack don’t get so afraid they sell their horse.
If you do have a “problem horse” and things have escalated to this degree with you, it’s not behavior only, it’s pain.
I would suggest you not get on this horse again until you’ve put a bunch of diagnostics into her, and if that’s too expensive at the moment then try to put her in a herd on a field for a year to relax and move around which can fix a lot of stuff.