Kissed another girl

I understand situations like these are vastly different depending on circumstances and the people. So I will try to give as much detail as possible. I was best friends with my girlfriend of a year before we started dating. And now we have been dating for almost a year with no speedbumps. We go to school 1200 miles apart. I love her to death and I would do absolutely anything to make our relationship work. But I'm lost and have no idea what to do. It's been 3 months since I've seen her. However we talk and Skype every free second of every day. And we are both happy. I was in love with her before we started dating, before she realized she had feelings back. And I think this still shows in our relationship in how much we love each other. And by this I mean if you were to tell 10 people what happened. They would all assume it was he because of how much I care about her. But I screwed up. I got sloppy drunk at a party last night. A few of my close female friends came over and one of their friends from out of town was visiting. We came back to our appt after a party. And they were all too drunk to leave. One of my roommates has a "relationship" with one of them. And they all spent the night. The problem comes with after having way too much to drink the girl came on to me. I'm not sure of there was chemistry or she was just making sure she had a bed to sleep in. But we kissed. She asked if this was ok when she walked in to my room and saw a picture of a girl with her arms around me in a frame on my desk. And I lied and told her we were having issues and I wasn't sure what I was going to do. I have no idea why I said this. I couldn't even imagine being with anyone else. Or doing anything to hurt her. But I did. We made out for a while and things escalated a little bit. But I stopped. And said were both drunk and went to sleep before anything worse happened. I layer there thinking about my mistake all night. I didn't get a wine of sleep. While I was laying next to some girl whose last name I didn't even know. It meant nothing. I'm never going to see her again. Is this something I should confess to my girlfriend. Am I protecting her feelings by keeping it a secret. Or is it something I need to get tell her and hope for the best. I'm not sure I can handle keeping it bottled up. I feel awful.

Edit: I don't want her to find out some other way in the future, and I don't want to ruin things with a girl I have a future with by building our relationship in top of a secret.

You definitly need to tell her if you dont then this relationship will never work. If she loves you as much as you love her than she will forgive. And if you love her as much as you say you do then youll find that you have to tell her