Another female perspective.
I cheated on a boyfriend in college, and then was cheated on in a long term relationship after college.
My now fiancé had dated a girl for 6 years and was engaged when his girlfriend cheated on him.
Some things for you to think about:
A) I would never ever ever cheat on my fiancé. The "once a cheater always a cheater" line is not true. I respect my fiancé and I love him, and he respects me and I know that he loves me, even when we argue or are going through a rough patch. I am confident that he was respectful and loving to his ex, but apparently she didn't feel he same way because she cheated. You don't do that to someone you really love.
B) I cheated in college because I was bored and the relationship was pretty much over. We were fighting a lot more than we had and I'm pretty sure we were both over it. I wonder if I subconsciously was trying to find a way out that was "easier" than actually talking about it. We tried to make it work for some reason but it ended up being a disaster.
C) My post college ex and I lived together too. Then he started getting secretive and sneaky and of course I suspected things. He lied about it for a looooonnnngggg time and it pretty much made me insane. She did you a favor by coming clean after only a little while. The thing that really hurt me the most was how long he kept it from me and how i had a gut feeling it was going on but didn't listen to myself. Lesson learned.
Long story short, I never thought I'd get over the feeling of being cheated on by someone I lived with, trusted, loved, etc...but I did and ended up meeting a fantastic person. The idea of either of us cheating isn't even something I think about now. We don't always get along and we aren't always lovey dovey but we talk about things when we fight and we always work it out.
You need to think about if this girl is someone you want to "make it work" with...with this situation always in the back of your mind...or if you'd rather meet someone else with whom you can confidently say this is not an issue and know it to be true.