I am going to speak plain here... unless it involves your children or a specific legal issue I am not sure you will get what you are looking for. I think a therapist might help you. The important part is to consider what your expectations are. Do you want an explaination? Do you want an apology. Do you want her to tell you what she thinks you did to "push" her into this affair? Do you need her to take ownership of it? Many people that cheat find a way to put some of the onus on their spouses for why they cheated. They do this to rationalize it or to make excuses for it or to make themselves feel better about being a sh*tty spouse. I am not sure what you are trying to accomplish and that might be something you need to consider. If you are divorced though and intend to move on, I think your conversations should be kept to the kids or a legal divorce related issue and not what happened or could have happened.
I'd like to say that I say this as someone who has been a friend to someone who has been cheated on, not someone who has actually been cheated on. Its possible that I may feel differently if I was in your shoes. Either way I am sorry this situation is so painful for you and I hope whatever you do brings you peace.