It's hard to let go of someone who's hurt you when you care about them.

However, I think I can safely say that this person does not care about you and that they do not deserve your affection.

No matter what goes on in her own life does not give her the right to abuse you, which is exactly what she's doing. She cheated on you, was cold to you when you needed her... she's using you and doesn't care about your feelings. And the Satanist thing, well. That in and of itself is more than enough reason to stay away from her.

Rather than try to hate her, simply understand that she does not love you and probably doesn't want to. She's a user, plain and simple, and all she will ever be good for is hurting you. It isn't your fault things came out the way it did... you can't blame yourself for the way others choose to live their lives. That girl is a bad person, probably rotten to the core.