Sorry to jump in - please -everyone - don't take paracetamol for calming purposes! oh my gosh that's dangerous and not how it works... (sorry, spotted comment as was reading thread)
I'm sorry to read about all this OP. I found out ex had affair, and when confronted he oscillated between a REALLY HIDEOUS justifying of it 'but she cares' 'I was on my own' 'she's nice'!!! to 'I never did that you're lying'
I actually wish I'd not confronted him at all, but I exploded as soon as I found out.
It killed off the last ebbs of 'love' I had for him, really shattered the illusion, but I gave him another chance
although my heart was not in it, we had a baby etc and logistically it was easier. However, this spurned him on to renew his contact with her and try to meet her more often not less -it only lasted about another year and I was actually, looking back, relieved to find out. It finally made me sure I was right to leave a very unhappy relationship. He still denies it to this day
and now says I had an affair
unbelievable.
Anyway - best advice, stay calm, cool collected on the outside. Rise above this mess he's made. You're not in the wrong, none of this is your doing, none of this could have been changed. If not her, it would have been someone else no doubt. Please don't beat yourself up about anything. Think smart - all the evidence, all the financial things in order, everything everything, any secret bank accounts as well - find out everything and take copies/originals of everything important for you and your DC future security. Then decide how to approach this. I would actually suggest not giving him the evidence - even going quiet about it if you can bear it, while working out what you want to do (and gathering more evidence). Watch what he does, what he says and that could help you decide and have no doubts left, that you can't possibly live with him/be with him again - lingering doubts make life miserable, and make divorces hard work. If he lies and lies still even though he knows you know, and has no respect at all to tell you honestly - I think it might make things even easier for you later on.
Best of luck OP and solidarity from the other side! 