I think writing a letter is a good start. I've always used writing to help me let go of things. I wrote many letters to my husband after he died, many out of anger for what he had done. On the two year anniversary this past may, I finally decided to forgive him for leaving me. I did that by accepting the situation for what it was, and understanding his decision.

Obviously your situation is different, as your person acted cruelly with no understandable reason. You may never understand why she did it, but that doesn't mean you have to suffer with it for the rest of your life. Maybe just saying well, she's an a**hole, some people are just horrible people and that has nothing to do with you.