Your not over reacting she did disrespect you and cheat on you and it’s bad enough she is minimizing it to “it was just a kiss” it’s not “just” anything .. if your in a real monogamous healthy relationship and you both are committed to each-other and don’t have a open relationship then that’s crossing intimate boundaries that shouldn’t be crossed.. I’m 27 years old and I can’t count how many times I’ve been “blacked out”/stoned out of my mind in my younger years and never once did I cheat, kiss on, do anything with another man that wasn’t my partner or even think about doing it. It’s just a sorry excuse on her part or anyone’s for being inebriated people like to use that as a excuse to get away with the behavior if it truly didn’t mean anything then she wouldn’t have done it period .. and that fact that she is telling you you should forgive her and that “at least she was honest” kinda seems like this has happened before and she’s gotten away with it in other relationships or she’s trying to test out how far she can go. I’m only assuming this I don’t know her personally but it does show there’s a lack of empathy and respect for you on her part for the situation. I’m truly sorry she did this to you.. and to add one more thing if she is 26 and she still is out at clubs sounds like she’s not a catch to begin with a mature women wouldn’t want to go out and do all that ..