How do I get my horse to stop biting and pinning her ears back at me?

I have only had this horse for about a year and a half. When you go up to her she most of the time pins her ears back. She seems really crabby most of the time. She has bitten me three times now, quite seriously, severe bruising and cuts. When I got her she was very thin and very bit up. She apparently had to fight for her food in a large herd. She is now very pampered by me and she is in a herd of 3 and very well fed and cared for. Each time she has bitten me it has been around food. One time was when I was putting her grain in her feed bucket. Another time was when I went into the corral area and I think she was defending her food. The other time it was when I went into her stall. Can this be stopped. I'm afraid she is going to hurt me or someone else very seriously. She is a bombproof trail horse and a very well bred Quarter Horse. Is there anything I can do, or should I get rid of her?