I ran into this same problem with a mare that I have. What I did was went with a chain bit with a very long purchase and took off the chin strap. She works like a champ now! Also I know you said you had her teeth done but have you checked for wolf teeth? These can be a huge problem especially in mares? Hope this helps. Also try a tiedown with a rope noseband and see if getting her nose sore helps at all?

After all medical has been checked as suggested, look at some of the english hackamores. They make some nice ones. Leather adjustable straps with wool lining. They have excellent contact but will not sore the nose. Some english bits do wonders for a "western" horse.
Hope you find the cause soon.
Kathy