there are a few ways to attempt to break this habit:
1) smack her mouth when she does it, this seemed to work for my 2 year old who has a nipping problem.
2) walk her either with her bridle or a chain on her lead rope and give her a slight, yet firm tug when she bites.
3) completely ignore her when she goes to bite (other than moving away from her), she MAY, and I mean may, just enjoy the attention she gets from doing it. 90% of the time horse owners seem to notice bad behavior more than good behavior, so ignore her when she does it.
When using 1 or 2, be sure to smack her mouth or tug on her nose BEFORE she actually bites, if you can get it when her ears pin back go for it, if you don't notice it til her mouth is open and leaning towards you that's fine but try to catch and punish her before she does it, it will get her mind off trying to bite you. Anyway you punish her, use a firm "no", you should get to the point where when you say "no" she backs away from you. My TB will turn from me when I say no, it doesn't matter if I'm talking to him or not, as soon as he hears the word he turns away from me and will sometimes walk away if he can.
Hope all goes well. Keep me updated.
16 year old TWH Mare named Ginger
3 year old APHA Paint Gelding named Fox.
RIP Evie (Clydesdale) 7.29.09 & Magic (OTTB) 2.23.10