Let it go... work on yourself. Give her all the uninterrupted time to heal she needs. Do not keep pressuring her on R. Every time you do, you are driving her further away. Just realize that her healing made lead her to a conclusion that you might not like. Accept that and move on with your own healing no matter what.
If there is a chance to R, you are going to have to rebuild your "house of trust" brick by brick. The foundation the house will be built on is you. A solid foundation means a solid house. But realize, you will always have to maintain your house. If you don't, you may end up where I am now.
I also had an A many years ago, the bomb dropped during our S. It was a while before we even spoke, texted, emailed or anything. When we finally did talk, I had to be brutally honest with her about anything and everything to do with the A. She grilled me for months about it and I agreed to be an open book and always accessible to answer any question she had. Your fiancee may not want to do that but be prepared for it. If you are faced with that, you would be best served by having your head in the best possible place.
Me: 43 Her: 37 D: 4 T: 20 years M: 15 years 1st Separation: 12/20/06 Drew up papers for D: 9/15/07 Reconciled: 11/1/07