TB's are exercised and hand galloped and work out to build up their stamina and strength. It may be that you are keeping her too hyped up by lunging her.
I'd switch to just mounting, and make sure too that she isn't being toed as rider goes up.
Could also be feed is hotter than she needs, or hay is.
She also may have dental issues, reins being held too tightly when someone is going up, dragging toes across back.
Video would help, both us and you.
And while you may have trained and been riding for years, rearing is a serious matter, even if she does not do it with you at present.
I knew a woman that had been riding many many years, Saddleseat on Saddlebreds, she was in the warm up area and as she tried to mount, horse came up, she stepped down and back. Horse was settled, she attempted to mount, horse went up, she stepped down and back.
Did it a couple times more, as was supposed to be heading into arena...went to mount, horse went up and as she stepped down and back? Her foot slipped out from under her, and the horse slipped, as it got to its feet, it put a hoof on her head and crushed it, killing her right there in the warm up arena.
Eventually your luck will run out, even if horse is not doing this to you presently.
A thorough vet exam with equine only vet would be where I would start.
And secondly, I would quit letting others ride her too. If it is important to you that others have a horse to ride, find one that isn't so sensitive. If this horse suits you, I would keep her for myself. Something others are doing is not agreeable to this mare. I can count on the fingers of one hand and have a couple left over, the number of people that I let ride my personal horses.
Horses make me a better person.
Last edited by Palomine; 01-20-2013 at 05:07 PM.