Did I ruin it?

I've been dating this girl for about a month and a half and shes great. Well, the other night I went out for a drink with my roomate and had a little too much. And the problem with that is I get very flirty and friendly. (I rarely ever go out drinking..maybe once or twice a year) Anyway one of my roomates friends starts to chat me up and gets a little touchy feely. She and I talk for a bit then all the sudden she starts kissing me. I was caught a bit off guard but at the same time...I didn't stop it.
The "bar girl" wants to see me again but I want the relationship I've been in. I feel completely awful and guilt ridden and I want to tell her...but at the same time I feel as if I should just never say a word because I know it won't happen again. So, did I just ruin a good thing here?

Don't dump that in her lap unless you're concerned it will come back to bite you later. If there's any chance she could ever find out, you should tell her what happened in a very diplomatic way, for instance:
"God, it was so freaky, I mean, we were just drinking and then she started kissing me. I was totally surprised. I guess I need to be more careful who I drink with because I'm totally into you and I want it that way" is much better than,
"Damn- I'm a stinking man-whore. I didn't even try to stop her. I feel so dirty. Make me clean again."

I agree with Giga. Keep it to yourself if at all possible. It will only make the girl you are dating feel insecure. Tell the bar girl to go away because you are dating someone.

Honestly, I don't think she would find out. We really don't have mutual friends and the only person that would let it leak out would be my roomate. I guess it boils down to a guilt thing, I feel awful over it. I'm just glad I didn't take her up on the offer of an "overnight". But thank you for the input. I think I'll just keep it under wraps.

Don't make your guilt become her problem. I mean, if you tell her, YOU might feel better, but she will feel worse.

BTW - this advice only applies if you are absolutely certain that you won't let it happen again.

I'm sure everyone says that.. "It will never happen again". But I'm sincere when I say this.

yea if you are sure that it wont happen again, then just leave it to yourself and treat it as an 'accident'.

There is no point trying to allow any space of imagination for your gf anyway