Being a lesson horse is not an easy job for any horse, and that may be a big part of the problem. For starters, they have to be able to tolerate unbalanced riders and inconsistent aids from one rider to the next. Some are better at it than others, some will never adapt. She may have a history of injuries or poor fitting tack which is or was causing pain. She may have been doing too many lessons every day. Barn staff may have had holes in their training, leading to problems handling her. She may have been mistreated. You don’t know her history of training on the ground or with a rider. It sounds like she has been bounced from one barn to another, one more contributor to her problems. If she didn’t work out in a lesson program she was sold - in her case several times to her detriment.
No matter how good a horse is performance-wise it doesn’t mean they are cut out for life as a lesson horse. If she was skinny when you bought her that says no one was taking care of her. Add it all up. This is one of those deals where most of the horse’s problems start with human beings. I expect she figured out years ago that she’s not interested in the job. She hasn’t had a person with the time, love, and patience to invest in her. It sounds like she finally landed somewhere where everything has changed for the better.
Everyone talks about retraining a horse to do different things that we want. It’s not easy to substitute a different response for one that is well-established. They never know in advance what we want so they take a guess: if they are correct they get a reward. If not, we keep going. That isn’t necessarily a good training technique. It’s easier if we unwind back to the beginning with groundwork they do well. If a problem crops up, go back to something they do well and try the cycle again, We need to keep in touch with the 3-second attention span. Repetitions are key to their learning style . If they need 100 reps to learn something you don’t have to do them in one day, or week, or month. Always stop in a good place.
My big Paint gelding is 27 and we’ve been together almost 21 years. He is versatile, smart, curious, and exceedingly handsome (he makes me say that). Everyone who sees him falls in love - most recently the UPS driver. He doesn’t do things because we’ve been together for 20 years. It’s because for the entire 20 years I’ve been say “good boy” constantly. I ignore what I don’t want unless it is potentially dangerous - e.g. biting or kicking. And I concentrate on having a measured response. I watched a woman’s mare break the crossties one day, and she got on her - “don’t do that” - going after her head. Horse was in a panic and broke the ties 4 more times in about 15-20 minutes. She’s known to have an attitude problem. I feel bad for her horse.
If I do something the same way once every day my gelding figures it out. I’ve tried to limit his training to 15 seconds a day, sometimes more, often less. It took me a while to shift my training techniques. I forgot one of the most reliable cues I have. I lightly hold the noseband on his halter and gently stroke his face. It’s a head-down cue that calms him down. It helped yesterday with the farrier. Horse was distracted activity outside where he couldn’t see it.
I have what I call my accidental training techniques. Do something exactly the same way, every day. One day I figure out that he has already figured something out. He’s ahead of me, I guess. Time, consistency, patience are the keys.