A quick kiss doesn’t sound like much. Really, the physical act of a brief kiss is harmless, like kissing your aunt or grandmother. The entire problem has to do with the circumstances and your feelings at the time. If you were intoxicated, out of control to some extent, and felt some attraction or lust, those are the sorts of things you need to deal with.
The guilty feelings you are now experiencing do not necessarily mean you did something gravely wrong. It could be a kind of scrupulosity.
I recommend that you try to figure out and reconcile as much of this as possible on your own. It is unfair to your girlfriend to recruit her to be your confessor and psychotherapist. What you need to do is think and figure out for yourself, perhaps with a trusted and neutral advisor, what sorts of words and acts should be reserved for a girlfriend, what behavior is appropriate around attractive young women other than your girlfriend, and how to moderate your drinking and stay in control of your behavior at parties. Resolve to do better in the future.
When you speak of this with your girlfriend, keep it short and simple. Don’t be dramatic. Don’t hyper-analyze yourself or the situation. And remember, it’s not all about you. Make it about her, and how you feel about her. And keep it short and simple. Best of luck!
