How can I let her go?

It has been 6 months since my girl and I broke it off. Why can't I seem to get her out of my head? I have 4 other girls chasing me now; why am I so attached to her? I know she doesnt care for me anymore because I made a huge mistake in telling her boyfriend about us. It wasnt intentional, but how can a person forgive that kind of betrayal? I havent even been able to forgive myself for this mistake, and I know it was a misunderstanding between us! I have accepted that I need to move on, but my heart will not let go. I am interested in other girls, but I dont want to committ to anything other than conversation. I still think of her every day despite the fact that we havent talked in months. What the hell am I supposed to do with these good and painful memories???