How can I get over the guilt of kissing another man?

Almost a year ago I began university, my boyfriend and I decided we would do long distance and we are still together now. However one night a boy kissed me and I kissed him back. I told him straight away what had happened.

I was drunk but that is no excuse. I came home and we resolved it and ever since everything has been fine. But now he has begun university and I trust him with everything but I keep thinking of what I did. I still feel so guilty and I just don't know what to do. Please help me