Look. You wanted to believe her. Thats all there is to it. You had already long ago decided for yourself who she was. When confronted with evidence that was not the true her, you didnt want to believe it. You allowed her to tell you that they "got to 2nd base". We dont want it to be true. Denial aint just a river in Egypt. I would also wager you had a difficult childhood with one or more parents/siblings that were manipulative, and you were always trying to please and never received their approval. Possible even abuse. Moving on, you wanted to be loved so much that you allowed yourself to be convinced she was "the one". Then you ran into the cognitive dissonance when the truth came out. This leaves you off balance, confused, shocked, and spinning in circles. You dont have a close net of supporting people (family, friends, etc) and may even feel stupid or naive.
Heres the truth of it. You are probably very empathetic. You also look to people to fill the void never filled by your childhood. You arent sure you really had a bad childhood and didnt deserve what you got and never sought real therapy. You selected a SO similar to what you grew up around. You want her "love" to make you fill whole.
The truth is confronting her is useless. She is an abusive POS. She uses you for a security blanket and does as she pleases. Then you are there to pick up the pieces, make her feel desirable again, give her an ego boost, then she goes trolling with a big net to find the new guy to boost her ego. Confronting her will give her the chance to wear you down, gaslight, manipulate (2nd base), and twist things around. It will magically become all your fault. You didnt love her enough. You were not emotionally there for her. It was your lack of XYZ that made her look to get banged in a hotel room to a co-worker and lie about. How can you be so selfish?
If you give her anything to latch onto she will do it. Her unwillingness to end the relationship is NOT a sign of her deep undying devotion to you. And when and if you back her into a corner (I find this unlikely) she will "agree" to go to therapy and marriage counseling. And she will use that (if she goes and doesnt pretend to create a fake fight to blame you for ruining her with the doctor to get out of it) to identify even more subtle ways of manipulating you.
There is no closure here for you. Some people suck. And there are many of them out there. Your partner in life isnt supposed to "make you whole". You are supposed to do that. That confusion is cognitive dissonance. The manipulating and lying? Gaslighting. This is who she is. Believe it.
So, do not confront her. Speak to a lawyer. Get your ducks in a row. This isnt going to be pretty. She will lie, twist, manipulate, and possibly even try threaten or abuse you. You have no idea what you are going down the barrel of here in your future. And holding off on it will only give her more time to cover her ass and screw you over. Be careful. Very careful. You arent stupid. You arent a loser. And you sure dont deserve this crap. Follow the guidance. Lawyer. Good one. Spend the money. Protect your paternity rights if that kid aint yours. Use anything and everything as leverage in this battle. Stay calm, cool and professional. Chill. You are really fighting for your life here. And it matters, dammit.