If your W is having an A, your bringing it up is going to push her away further. Especially, if she is at the "we are just friends" stage.
It would be best for it to be exposed by an accident or a 3rd party. I read that once an A has been exposed that some of the luster of the excitment will be worn off. Also, the average A only lasts 6 months.
You are going to have some really depressing thoughts ahead. You will be imagining all kinds of things your W might be doing. Try to get a STOP sign in your mind whenever you think these thoughts. Try to think that the lady you feel in love with and made vows together with has been taken hostage inside herself. Somewhere in her mind, she knows what she is doing is wrong.
Read the DR chapter on infidelity.
I feel for your situation and I am sure you probably wish this was a bad dream you are going to wake up from. It would be kinda nice to have a remote control to fast forward through this part of your life like in that movie "Click".