I screwed up but should I tell him?

this has been eating me up the past 2 days I went out with a few girlfriends to a bar saturday night and got pretty much hammered. My boyfriend was out of town on a bachelors party so I figured I'd go out. Well, at the bar we run into some people we went to high school with. I graduated 5 years ago so I hadn't seen them in years. One was my ex-boyfriend. Anyway, all the other girls decided to leave but I was having fun with the other crowd. They offered to ride me back to my apartment later.

Well, last call ended with me and my ex by ourselves so he had to take me home (about 2 miles). He ended up coming in for a drink. we joked around and he started tickling me. Then he ended up on top of me tickling me. Eventually our faces got close during so we started kissing...for about 20 minutes. Then I got scared and said he had to leave.

He keeps calling and I won't answer and my boyfriend came home sunday night and was wonderful. The thing is he'll break up with me if I tell him what happened. But I feel so bad. We have been together for 3 years. It was just being drunk and those old feelings came back cos I was drinking. I don't feel that way sober. I feel so bad but I don't want him to stop trusting me or it will end one day I'm sure.

Can I just let it go or will it eat me up inside or will he be able to forgive me over time? I really screwed up!!!!!!!!!