How do I let the anger go?

Monday I was supposed to see my girlfriend when she got off work, she said she was tired and wanted to just go to bed so I shouldn't bother coming over. The next morning I get a phone call from her where she says she so sorry but she cheated on me with a guy from work, she said it was a mistake and it will never happen again. The remorse I saw in her made me want to believe her, I want to trust her because I love her without any doubt in my mind. I want this to work out but I can't stop being angry, any second my mind isn't occupied it flicks to that night. Where were they hanging out at before they had sex? Did she just invite him into her bedroom or did he start it? Why did she invite him over and tell me not to come? Was I not enough for her? What if the sex with him had been better? Would she have told me or just kept it up behind my back? These thoughts pummel me every moment of every day and I can't stop them or change the emotion I feel about them. How do I let go of the anger and hurt so that we can heal together..... Is that even an option.......