My fianc cheated. How can we mend our relationship?

I'm in a relationship with my fianc and I currently found out that she's been cheating on me with a co-worker. I was very upset none the less. I understand what she did was wrong, but I want to spend the rest of my life with her. So I'm willing to forgive and move on. We have been together for a little over 9 years, and we have a son. At the beginning of our relationship, I was unfaithful and she caught me cheating. She forgave me for what I've done to her and since then I been completely faithful. I'm worried about our relationship and want to move forward but its been very unsettling. All I can think about is her with another man. I don't know what to do or where to go for advice?