Leave cheating girlfriend?

My girlfriend of 5 months yesterday admitted to me to having kissed another guy (who she had a sexual relationship with before she got with me). She assured me it didn't go further than that. This was in the first month of our relationship. Her explanation is that she didn't really know how serious our relationship was going to be so serious (we have been looking at houses together). She broke down yesterday and admitted it and was really very emotional about it. Thing is that I had asked before to her if anything happened between the two of them and she told me nothing happened but I had always suspected something happened. I do love her though and I'm convinced she loves me too. I don't want to lose her bit at the same time how can I trust her ever again? Any advice is welcome. What are your experiences with this?