<human>: Condense the text into a few key points

My SO [23 F] and I [26 M] have been dating for 2.5 years now. She cheated this past weekend out of the blue.

Backstory: I got a call Monday with my SO bawling saying she messed up and didn't mean to hurt me. She got drunk Saturday night and ended up at an old friend's house and they had sex. She says she stopped it in the middle because she realized what was going on. I told her not to contact me and got off the phone.

Confusion: Our relationship was perfect. I saw absolutely no sign that this would ever happen. That is what concerns me most because how would I ever know in the future? There are only two reasons that I'm even holding on and considering this usual deal-breaker to be something I want to work on. 1.) She was the one to call and tell me. 2.) She just got on Ativan and I know mixing that with drinking can make one blackout. 

I've read enough infidelity forums today to know that most replies will be "she couldn't control her alcohol so it's no excuse" or "obviously this was an underlying desire." I think there are rationales for each side so I'm just trying to wrap my mind around this.

I've never had this happen before. She was the love of my life but I'm not some desperate guy thinking she's the only thing in the world. I realize I CAN move on, but I haven't decided if I should give her another chance.

Can anyone shed light on the situation and/or give advice?
<bot>: SO cheated once after drinking on new benzo and then told me of it. How should I make decisions moving forward?