No story is the same, but here's mine.
DDay 1, I confronted WH after finding some flirty emails on his phone from/to a co-worker. First question: Did you sleep with her? Answer: No, I have always been faithful to you. 2nd question: Did you kiss her? WH: Yes.
Lots of crying, hysterical bonding, establishing NC with a person he still worked closely with, therapy, etc. ensues. I often ask if he's lying. Often. I ask him to tell me if his penis has EVER been anywhere I don't know. I probably asked 1000 times. Every time, no. Of course not. You are the only person I've ever been with, the only one I have ever trusted.
TWO YEARS GO BY. I've lost 20 pounds, losing hair, PTSD symptoms, can't kiss him bc it's what he did with her. But our relationship is really getting better. We fell in love again after 20 years together.
DDay 2, he confesses to sex with her. He said it was because he'd evolved so much that he knew letting me have the info and the truth no matter what happened, that I deserved it.
A few days later I had a time, and a few weeks later I had a poly to back up the timeline and whether or not he planned on leaving.
INSTANTLY after I knew the truth, that nagging feeling went away. It is INSANE how our bodies just know things. I HATED his car that he drove during the A. After DDAy 1 I didn't want to be in it (even though I was). I had asked if she'd ever been in it and got a no. Turns out he'd gotten a blow job on the way home from the business trip where they consummated their relationship for the first (x6) time and then had sex with her in that car a few days after that. I actually cried when he was looking for a new car and thought about getting the same car, but a newer model. I just HATED it. Even though I didn't know anything physical had happened in it.
(Interestingly enough before spontaneous DDay 2, I also had dreams (multiple) about finding out that he slept with her. In one, I went and had revenge sex with the other BS in her home and we videotaped it and sent it to them. How messed up is that? I'd never had a dream like that before or after.)
Anyways, long story shorter is that I think a poly is a good idea. That nagging feeling of mine went away after I got the truth. TBH I didn't even need the poly. He was vomiting truth (even confessed to a bj that he received while we were broken up in college for a semester) and had no reason to lie. I had always told him that I would leave if he had sex with her. And I probably would have that first day. (Which is why he colluded with the cOW to keep the secret, he knew I would make it VERY public about why I was leaving, so he was mostly covering his own image and his ability to keep working at the company - she is also very image conscious/had a family so she had a reason to lie as well once she figured out my WH wasn't leaving me for her.)
You truly never know what you'll do, but the truth feels good. I do feel at peace with that part. I feel like I know the truth. And whether or not my marriage can survive now? Only time will tell.
I'm sorry you are in this situation and hope you get some peace.