We're three years in. It was tough, really tough, but we're getting there. I should add that on the back of the emotional affair he shagged her. He says once, but I imagine a couple of times - after all, I'm not stupid and the story started with the lie that they hadn't slept together, so make of it what you will.
She lives in North America and most of their affair was carried out online, apart from the two-day 'business trip' when he went to visit her. (She was one of our sub-contractors....).
For me, the emotional affair part was much harder to come to terms with than the sex. Although I know I would have been hurt if he'd had a ONS it was the emotional involvement that was the killer. The discussing of our marriage, talking about our children. Oh, and the complete character assassination that she carried out on me - that he started to believe. She had never even spoken to me, let alone met me. My contact with her was through half a dozen emails to set her contract up. He was such a gullible idiot.
He's not the same person that I married. I still love him, but it's very different now and I don't trust him in the same way - he shattered that. I've grown from it though and have learned a lot about myself.
We've been together almost 30 years and married for 27. If it had been at the beginning or even 10 years in I'd have walked without looking back. As it is, there's a huge emotional investment, children, and we run a business together. He's not actually that bad really. Just this once he was led by god knows what - his dick probably.
I wish you luck OP. You have to do what you want. Make choices based on how you want this to pan out.
One thing I would say is that many people on here will suggest you LTB. Only you can make that decision - it's not wrong to stay if you want to.