toomany - that's exactly is HE created this mess, not you. This is not your fault. No matter how much your head spins with it all, this isn't your fault.
It's not like slipped with his cock out of his pants and just happen to fall into her FFS.
He willingly had sex with her. More than once!
It's so bloody hard to seperate your head from your heart. No-one expects you tp just switch off from your feelings but maybe, with this space, you wil start to see with more clarity.
Well done for not changing your mind on him going, that must have really upset you.
No doubt you'll be thinking things like 'What if I do end it and he sleeps with other women, and not me anymore?'
'What about if I try this or that, maybe he'll stay faithful, I'll be enough then won't I?'
'Perhaps we could go on holiday and take the pressure off the relationship, come back and start again?'
toomany - you know that none of the above will ever fix this, you'll just be papering over the cracks.
This bloody structural, not cosmetic. Smiling on the outside, shattered inside 
Use the next few days to think about what YOU want. Take everyone else out of the equation, even the DCs, and really think about how this will ever be the same as it was......
It won't will it?
You know why he didn't answer you about the tables being turned, it's because he'd through your sorry ass out and then tell anyone who would listen what you'd done to him.
I doubt he'd forgive and forget.