I also agree with Anebel. Once you have eliminated health issues though, I would take her behavioral problems by the horns and shake the heck out of them.
If a horse bites, no matter how old, young, abused, injured, or flighty it is, I will hit it. Hard. On the nose. This startles them enough that they will not want to do it again.
After I give them a whack, however, I will go and rub all over there head after a moment or two. They WILL try to pull away and WILL act frightened. But, by rubbing them then after they will see that the discomfort (smack) was a result of the action (bite, or even kick) and not because people are evil.
I've smacked my rescue and my current colt basket case. My colt was most likely abused formerly, but biting (which he did!) is not tolerable. He also got the surprise of his life when he swung his butt at me today to. Thwack! Won't be trying THAT again.
In my honest opinion, he better respects me now after being corrected for his incorrect behavior than before. Horses need to know who's the big cheese, and by golly, I am!
(even though I'm over a thousand pounds lighter than them XD)
Wait! I'll fix it....