I struggle with this too. I always play out the same story, try to fill in the blanks, from what my fiancee told me:
My fiancee has told me when she cheated on me she went to a party, slept with some guy she met that night.
She was angry and gave me all the details. When I asked her why, she said "I don't know... he was looking at me like I was prey and it turned me on" - something to that effect. They were playing beer pong, it was some college type party.
What kills me is that she made me wait like a month, 4 dates or so, before we became intimate.
And with this guy she just gave it up to him at some random party, meeting him the first time.
The worst thing is she told me "I got tested after, and everything turned out OK, I don't have anything" so she didn't even use a condom.
And if you met her she seems like the most innocent good girl you've ever met, not flirtatious or provocative at all.
It's kind of a mind-f*ck.
I found this out after 4 years of being with her, living together etc...
I totally trusted her but after finding that out, I ask myself if I even know who this person is....
Which is why I'm here. Because I thought she was the love of my life. I'm trying to figure out who this person actually is. And whether I should try to make it work, or risk not finding that type of connection again. (This isn't my first relationship so I know how hard it is to find a true connection).