How can I forgive and let go of a cheating ex?

The relationship started with her "accidentally" making out with someone else. She would always tell me she never fell for anyone the way she fell for me and had the most believable justifacations. I, stupidly in love, fell for it. Next, she did ual favours behind my back for ciggaretts...and then I fnd out she has been using heroin behind my back as well as other substances that conflict with my new way of life...yet I still loved her...and I unfortunatly still do...I wish I didn't...I try to let go, forget, forgive...she is still on my mind, haunting me with painfull memories and with the "hope" that someday we have a chance...I want to get rid of all hope, all attachment, and ESPECIALLY resentment. I feel like this chick is tattooed into my heart and my mind, and quite frankly it annoys the hell out of me. Any genuine and respectfull comments/advice/answers will be much appreciated.