To me it sounds like trust is a bigger issue than what she did. It sounds like she violated your trust, and lied to cover-up something maybe she was ashamed of. She is the only one who can put your mind at ease and trust is an important part of a healthy marriage. Disclosure is usually the best way to rebuild trust. I'm sure she had reasons for what she did, but only she knows what they are. Maybe this was a one time thing and she did it for the experiencence. I think by lying about it to you, it shows that isn't someone she wants to be.

Being lied to sucks, but if she told you the truth back then, and you left her you would have not had the years you spent together. It wawasn't fair she took that choice from you, but is has happened. People change along with thier values. I don't think you should expect her to be the same as she was back then. I think the best thing to do is talk to her about it and try to work things out. Holding onto this only builds resentment towards her. My suggestion is to try and talk things out with her.