I've been in a slightly similar situation but different (if that makes sense?) I'll try to keep it short.
My SO and I have been together for 5 years, but the first year was the roughest.
We met playing an online MMO and he was having some personal problems with family (grandfather was diagnosed with brain cancer, then passed away) now right around his passing (October of 09) I noticed he was kind of getting distant, it worried me. I wasn't sure how to help him cope what happened with his grandfather (I was only 18 at the time and still in high school where he was 25 much more maturer than I was) since I wasn't helping he went to someone else, and well long story short two weeks before my birthday in feb I found out that he was cheating on me in an online relationship for the past 4 months.
Reluctantly because he knew what he did was wrong (I remember him saying "this is probably the stupidest thing I've ever done in my life) and I knew due to the circumstances in which it all started was because I wasn't being attentive. I'm not good at consoling people when they've lost a loved one- I try but it doesn't always work out for the best :/ but we decided to take a break. we didn't want to break up- neither of us did but it's what he wanted to do.
During that "break" nothing changed. Absolutely nothing. We still talked to each other frequently, we still played games together, even had a sex life. We don't even consider it a break now, we got back together 6 months later after he realized while he said "break" he never followed through.
Now, to be fair- how is this different than your situation? It was all online between him and the other person so it was easy for me to forgive him.
But with whats going on with you, maybe you guys just need to talk it out more? Maybe you both should try a break and not cut off contact?
Point is- this doesn't have to be major if you both love each other and she realizes she did you wrong and is willing to prove to you that she deserves your trust back, which can be hard to get back. It took me a few years to be able to fully trust my SO again but I've learned to forgive him for his lapse in judgement. You can make it a bump in the road and just try and get back to where you were, it might be a little hard, you might want a week or something to give yourself some space for thinking and deciding on if this is going to be a pattern but I think if you both love each other you will find a way to work it out.
We managed to :) I'm moving in with him next April, and I hope you do whats best for you.
The best of luck!