I cheated on my girlfriend

I cheated on my girlfriend which have been with for almost a year now. We decided to get married by the end f the year and all that. I's a VERY serious elationship.

I went to a party and ot drunk, and cheated on her with a girl i've known for years. Now i feel like SHIT. I can't believe this happened,and I know it never will. I never cheated before, and I can't believe how wasted I got. I'm afraid to tell my girlfriend, because we have to much at stake. I eould kill her, if I told her. I am really sorry for my actions, but I know it was a huge mistake. I jsut cannot tell her. No way in hell, I would loose her. I will just have to live with my guilt. I hate it, but I hope time will take it all away.