Horses don't understand human social structure. They only understand horse. She views you, not as the leader of the herd, but as her playmate. As an alpha mare, you need to educate her that the behavior is not appropriate. There are never teeth allowed on me from any of my horses. Lips and muzzles, ok, and only if they're being respectfully curious. I am the owner of my space and my person. They must move away from me when instructed, it's not the other way around. Youngsters will test - they have to, it's part of their growth and learning herd structure. Each of the babies has to try putting teeth on me at some time, and some event decide they should try to charge at me once. Lucky for me, I've seen that all before and know how to very swiftly shut down that behavior and move them without hesitation so they realize I am the leader. They will sulk and pout but eventually will come back again in a much more respectful level of interaction.
Black and white, clear boundaries and clear instructions, little gray area make happy horses. Oh, and I do feed all of my horses by hand as well!