<human>: Let's preface this throwaway account post with I (28/m), I've been seeing a girl (24/f) for about three months now. I can't necessarily say that our relationship has ran the smoothest of courses. I started out really liking this girl. She's smart, we have great sex and she's very fun to hang around. About a month and a half ago, we had a conversation about not seeing other people, and agreed that neither of us would. (She's currently out of town, and has been for a few weeks.) However, a few days before she left, in a drunken stupor she kisses some guy in front of me. That action really took me aback, and sort of took the wind out of my sails. I decided to forgive her, then she flies off for what will amount to about a month. 

A couple of days ago, I was walking about town with a buddy, and ran into a cute girl that I'm friends with. We all agree to grab some drinks before he has to go to work. We end up drinking for a few hours, and he goes off to work, leaving the two of us behind. Said girl ends up at my apartment and we had sex.

I'm currently trying to figure out how to address the scenario. I selfishly feel like I allowed myself to get into this situation because she kissed another guy, right in front of me. This sort of allowed me to say "fuck it," lower my inhibitions, and sleep with someone else. I enjoy the company of the original girl I am dating, and am considering not telling her what happened. This is the first time I've ever hooked up with someone other than the person I'm dating. I've read from other similar posts mixed responses; some people say carry this to the grave, some people say tell her and let her make the decision. What sort of opinions do you have?

I've never done anything like this before, and never thought I would. I don't want to do anything like this again, it's a pretty shitty feeling.

Can you condense the text into a few key points?
<bot>: I'm dating a girl, had a rocky start, and slept with someone while she was out of town. Debating whether telling her or not.