How can I get my girlfriend to forgive me?

Ok, this one is complicated. My girlfriend left me out of the blue in January after 5 years together. She had gone home for Thanksgiving and Christmas to spend time with her family. I was a little lonely during this time especially at Christmas and New Year, but we had been having a few problems which I thought we had resolved and I wanted to give her some space and time with her family to think about what she wanted. All the time she was gone she told me that she loved me and was coming back and she wanted to make things work, then a few hours before she was supposed to get on a plane, she called me and told me she loved me, but she just couldnt live in the UK any more, that she hated it and she was sorry that she told me otherwise. Despite this she did some more thinking, decided we could work it out and we got back together in Febuary and she came back to the UK. I welcomed her back with open arms and every day since I have felt myself lucky and recently she has been much happier than I have seen her in years. I have been a little insecure but I have dealt with it on my own for the most part. She never stopped loving me and I never did anything wrong untill recently. Now to the reason I need forgiveness. After our split in January there were a few weeks of bitter communications and then I cut contact for a while. I was completely devestated. I couldnt believe I was never going to see her again. I couldnt go to work for a while and I stopped eating. I spent most of my time in bed. We had been e-mailing but they had degenerated into bitter little messages about how we were hurting and we were both too broken up to talk on the phone. When we did it was almost like we were together again untill the call ended and the bad feelings welled up again. At our worst point, when I cut contact completely, I was angry and I went out. I wanted to feel better so I went for a drink and I ended up sleeping with someone. This is not something I usually do and I immediately felt guilty and completely disgusted with myself. After a few weeks of not speaking me and my girlfriend got back in touch and began to work things out and she started talking about the possibility of coming back. I wanted to tell her what I had done but I was too afraid that it would make her change her mind. I was miserable without her. I knew she still loved me and I knew it would hurt her. So I buried it and tried to convince myself that it didnt happen. Things were rocky to begin with but have been getting steadily better since she came back but she kept telling me how much she loved me and how much she trusted me and the more she said it the more I felt like a liar and a hypocrite. When I kept this from her I stopped being the trustworthy person I had always been. Now I am something I never wanted to be and I did it to myself. A few days ago I broke down and told her the truth. I don't know why, and I wish I hadn't. I feel like I have hurt her to relieve my own selfish conscience, and I have ruined something that was really good. I have never seen her so upset. I know she is not as upset about what I did as the fact that I kept it from her for so long. She says it would have hurt less if I had just told her before she came back. I didnt because I was afraid. I should have trusted her to forgive me at the time, but I was desperate to get her back and I didnt want to jeopardise it in any way. What is worse is that I know she has been feeling guilty about putting me through so much when I didnt deserve it. The truth is I really did. She says she probably isnt going to leave me, but just picking up and leaving isnt a simple thing anyway. She has build a life here and has nothing back home, but she needs to stay with me to remain in this country. I know I am the only person she has to talk to, she doesnt have any close friends that werent my friends before we got together. Since that day she has been civil and friendly, but also distant. I am distant too. She has listened to me while I apoligised and tried to explain myself, but told me words dont mean anything. Now I have nothing else I can say. I'm too ashamed to talk to her and I dont know how to even start putting this right. I feel like I have to wait for her to be ready, but I dont know if she will ever trust me again and I dont want to be pushy. I did a very stupid, selfish thing and I have put her in a very difficult situation. I dont know if she will ever forgive me.