I don't want to upset her as I am worried that she will leave me for him. I would do anything to have her back exclusively.
Fuck. That. Shit.
Listen, that approach will not work whatsoever. In fact, she will feed off your pain. Your anguish and pain are seen in her twisted view as "guilt" you should be feeling for "pushing" her into this affair. She is doing something she knows very well she would never want anyone to do to her and she wants to absolve herself of any of the guilt and shame that comes with it so she will just project it on to you.
You don't want to upset her? Well, yes, I agree, you should not "upset" this wonderful, compassionate, and thoughtful woman.
No, you should pull the rug from under her fucked up bullshit fantasy. Upset her? No, you should legally keel-haul her via the most aggressive, money-hungry divorce lawyer you can find.
Look, there is NOTHING you can say or do to "nice" her out of this. The only thing that has a remote chance of stopping it is to introduce real, undeniable consequences for what she is doing.
Let your anguish and despair turn to righteous anger but use the anger in a controlled and calculated manner. Become steely, cold, and determined. Nice, simply does not belong here anymore.
File for divorce immediately. Now, that does not mean you are getting divorced. But, you must file for divorce and have her served with the divorce papers - preferably at her place of employment.
Filing for divorce sends strong messages that she cannot project away, delude away, or ignore. It is a hard, cold, and very REAL consequence for her selfish and cruel actions.
Filing for divorce sends the message that you will not tolerate her fucking around. It sends the message that her value to you has plummeted - that will really jolt her. It sends the message that you value yourself far more than her selfish, weak-minded and dishonest bullshit.
It sends the message that you deserve better, and can do better than her and her dysfunctional bullshit - and she will know it too.
Do not cower in the face of this. I know how you feel. Hurt, confused, and debilitated. There IS anger that needs to come forth and stir up strength and aggressiveness.
Believe it or not, your strong response to her juvenile-minded bullshit will actually increase your value in her fucked up eyes. Right now she has devalued you to the point at she doesn't give a shit that she has crushed your heart and hurt you in a way she wouldn't do to her worst enemy. That's why you should take a very strong, proactive, scorched-earth response to it.
My ex-wayward wife was as far gone as it gets. I had given her chances for forgiveness and reconciliation and she just kept up her twisted perpetual honeymoon fantasy with her shitbag adultery partner. It wasn't until she was served with the divorce notification that she ended it and I got the tearful calls and pleas for reconciling, how she still loved me, how she never wanted a divorce, and wanted her family back.
I was too far gone and could never trust her again. We are now divorced and she STILL wants to reconcile.
THAT, is the power of real consequences.