How can I stop loving my best friend now that he has a girlfriend, even though he's cheated on her with me?

My best friend and I go way back. We've been eachother's most faithful companions for four years, eachother's confidants and partners in crime, and occasionally we even were friends with benefits. This year I finally admitted to myself that I loved him. All of a sudden, when I was on vacation for a week, he started dating a mutual friend without any notice at all. I was heartbroken, I felt alone and useless and replaced. I also feared that my depression and anger would drive him away even further. I felt like our friendship had been a waste. Then, about 3 weeks later, he and I had a very sordid 1-week affair. I broke it off, feeling confused and horrible and knowing that he felt the same way and wanted things to be easy again. After mush inner struggle I finally told him to go ahead with his life and his new girlfriend, and that I would try to salvage our old friendship. Now, I'm just afraid that's not possible; it hurts me so much to see him with her, and his heart isn't in his friendship with me anymore. I am lonely, angry, I feel used. How can I stop loving him through all of this? He was my life, and in such a short time everything has changed. I want to hate him but I can't. How can I stop loving him??