<human>: Can you summarize the given text in a few sentences?

This is my first post on here so please go easy on me. I understand before going into this that I am young but this really hit me hard as we have been dating for almost 2 years now. So we've had a couple rough patches in the past in terms of cheating but nothing severe, everything was forgivable. Two days ago she told me that while she was at a party a couple weeks prior, she had gotten way too drunk and had sex with another guy. She had no clue who this guy was and couldn't even tell me his last name when I was asking about it. She said she doesn't know how it even happened and can't remember much about the sex at all. I'm torn between being mad at this guy for having the audacity to do this (she told him she had a boyfriend but he completely ignored it). I just can't fathom how she couldn't stop right there and say no. The other half wants me to be mad at her for being able to do this to me. How should I handle the situation? I've told her that I'm willing to give her a single chance and if anything remotely intimate happens with another guy I'm going to leave her. My main problem is that I'm not sure if I'm ever going to be able to look at her the same. I can't get these images out of my head of her being pleasured by this guy, just a shadowy face in my mind that plagues my dreams. I barely eat food because I've lost all appetite whatsoever. Please any advice would be amazing.
<bot>: My girlfriend of 2 years had sex with another guy while extremely drunk at a party. He told him she was in a relationship but he ignored it. I'm not sure who to blame or how to feel, please help.