MF married couple, late 30's. Been together in total 18 years.
My wife admitted to me a week ago that she had "fallen for someone else" and explained without realising the term for it she wanted a parallel poly relationship with both of us, with her as a 'pivot' i think it's referred to? (Obviously i'm new to this) where never the two shall meet, she has me in the domestic home, then goes out to see her girlfriend.

The issue is she had pursued and initiated intimacy with the woman she's interested in behind my back for at least several weeks. In terms of activity it's allegedly just been kissing and touching above the belt, who's to say - fundamentally the issue is the same.

I give her credit for coming clean and telling me what she wanted.
But obviously trust has been damaged and is now an issue, and in the week that's passed since she broke the news she has basically tried to manipulate me into allowing her to have what she wants.
She has admitted this was the case and admitted to further activity i didn't know about when we had showdown talks yesterday.

We're currently on a break and I'm trying to figure out next steps.
I love her and we've come so far together, but i'm feeling very conflicted about what to do now.
I am conscious that she hasn't had much sexual experience besides myself, and i had told her previously over the years that if she wanted to address that to simply let me know and we could have that conversation.
I want her to be happy, I just wish she had told me before doing anything.

Any advice appreciated