You'll note: there are many of us here who have been cheated upon. Like GGrass, I cried every day for MONTHS. I lost 25 lbs. in 6 weeks. I didn't sleep, couldn't eat, and was a mess; but I was still expected to get up every day and be a mom and brave the world. It was the hardest thing I have ever done and I've run 5 marathons, including one with a blister on my foot from mile 2 onwards.
All the while, my H was telling me that he had "found a new lid to his pot." I listened to him tell me that this co-worker who is 13 years our junior was a very special friend, and that she would be there for him if I ever got cancer. (SERIOUSLY - these are the fantasies these two created to rationalize their relationship.) He wanted to buy her a car, a computer, save her family vacation home. My mind was reeling with the absolute insanity of what he was explaining to me, and he simply would be flabbergasted that I didnt get it. We don't tell these stories to you to pile on the guilt, but rather to realize that your ex-fiancee is likely feeling VERY VERY battered and bruised. She should trust you again? WHY, exactly?
And the answer to that why lies within you.
Are you willing to work to become a man that she can trust again? Are you willing to be patient with her? Are you able to show remorse? To learn to communicate with your words and to control your actions?
You have work to do on you, and this is really necessary before you consider really trying to win her back.
So - the FIRST thing you should do is work on you. Detach from her. If she moves on, as you fear, well....that is a natural consequence of what you did.
If she doesn't, and time allows the two of you to reconnect...And for her to see that you are a changed man...And for you to allow her to be angry and fearful and confused before she forgives....
Well, then perhaps you have done the right work then on yourself.
Like Ggrass, I think I would rather light myself on fire rather than trust another man. Here I wait for my H to get through MLC....Hoping that this is better for our children. Hoping that he'll come around and realize the utter devastation that he has caused in his formerly very happy wife. I can't expect anything from him. I can only try to become happy again on my own. Only try to be a better and stronger person for myself. Will I ever trust again and know romantic love again? Time will tell.