How can I tell the woman I love that I was unfaithful?

I am a 22 year old male and I have been with my girlfriend for almost 2 years. We have known each other for 8 years. We started with a long distance relationship which I was always faithful in. I am strongly against cheating. However last night a woman I dated before my Girlfriend and I got together was in town visiting. We remained friends after dating but never crossed the friend line. We went out to the bar and she was flirting quite a bit. I enjoyed the flirting and played into it but I had no intention of doing anything. At one point she attempted to kiss me and I moved quick but she was able to nip my lower lip. I basically gave her a lecture about how that was not acceptable. As the night progressed we drank a lot and her friend she was staying with offered to let me spend the night because I was extremely intoxicated. Now I do not remember a lot of what happened next. All I know is that somehow we ended up beginning to have sex. It was as if I was having a dream but as soon as I realized what was going on I jumped out of the bed and put on my pants and left. My girlfriend is out of town for a week and before she left we discussed not talking this week while she is gone (It was an idea I had for us to work on being more independant of each other). Now I have a week to figure out exactly how to break this news to her. I am very worried about losing her. I love her with all of my heart and I cant believe that I would allow myself to do something so selfish. I keep trying to tell myself that I did stop the act when it was going on but that is not enough for me to respect myself. How can I gently tell my girlfriend about my horrible drunken mistake? We have actually discussed in the past that if either of us ever cheated we would let each other know and work through it. I am a big believer that being drunk isnt an excuse because all it makes you do is act out thoughts you have had. I am also very against cheating and my girlfriend knows this. Now as I play back the night in my head I see that this girl I used to date had this planed from the get go and I cant believe I fell for it. I just need a way to tell all of this to my girlfriend and not sound like I am not taking responsibility for my mistake, but also i need to admit that I was basically seduced. I knew her agenda but I thought I was smarter than that. I wanted to stroke my ego by having someone flirt with me and I played with fire and got burned. Please help me to hold onto the love of my life. Thank You