Nearly 15 years ago. Mumsnet was around but sadly I didn't know about it... so I've been imagining how the thread would go if I'd posted back then!
ExH decided that the night before I went away to take a relative for a big cancer op was the time to hook up with my friend (his coworker) and then dump me. I had no choice but to go and be with my family member, but they spent the whole time having romantic dinners, dates, and nights together in our home. I was so devastated that I foolishly jumped at the chance to get back together a few weeks later when he changed his mind. Except that he kept sneaking back to continue the affair whilst convincing me that it was all paranoia on my part and that he wouldn't hurt me again. Ex-friend invited me over to talk one evening. It began as apologies but turned out to be that she wanted to tell me (why?!) that my husband had been seeing her again (again). It was clear we'd have no hope staying in the area, plus I wasn't coping at all and was a nervous wreck.
Moved away together to start afresh and began marriage counselling. He was very emotionally distant and openly pined after OW to me (I stupidly tried to comfort him and help him 'work through it'). All the while he was still expecting sex, which he occasionally got aggressive about or ignored my boundaries and did things I had said no to - often whilst drunk, meaning he couldn't remember the next day and conversations about what had happened went nowhere.
One day I lucked out on discovering they were still texting as left his phone on the side (I hadn't been allowed to touch said phone in months). That spurred him on to show me a secret email account used for them to communicate, including emails where they were laughing together about their boss asking her about her love life (if only he knew, chuckle chuckle), and others planning to meet up that weekend. I was gutted. He refused to leave her a voicemail ending things as he didn't want to hurt her. My mental health was on the floor, I was suicidal and felt like I was losing my mind. He thought I was faking it to manipulate him or get attention. I ended up very stupidly kissing someone else (inexcusable as I hate cheating), which I felt awful about and fessed up about straight away in order to do the right thing. He became hysterical, couldn't believe how hurt he felt and now 'realised what he had put me through'. It was very weird. We sat talking and he suddenly started crying and masturbating which I found utterly repulsive and was a key moment of the scales falling from my eyes.
Back we went to marriage counselling. Then I discovered that after my confession he had become very suspicious of me and had cloned my email account so he could spy on me (he had changed settings on my email account so that every time I received or sent an email it also sent a copy to another secret email account that he'd set up). There was nothing to find but it felt very intrusive. By now I felt nothing but repulsion for him so I used marriage counseling as a platform to end things and moved away asap. He begged and cried loads for another chance, then i heard he'd got someone else pregnant a couple of months later (a big blow as we'd been struggling with infertility since a good while before the whole saga began). While she was pregnant (not sure if he knew about pregnancy yet or not) he rung me at work to tell me it would never be over between us from his point of view.
The whole thing sends shivers down my spine just thinking about the horrible dysfunction of it all. I didn't tell anyone a lot of the details until after it ended because I thought we'd stay together and I didn't want people to think badly of him. If only I'd had MN as a sounding board it could have been a lot less painful! 
Thankfully the next relationship I had was with a very decent and lovely man, but he had to be very patient while I found it very hard to trust for a while.
Anybody else have a horror story that in hindsight MN could have diverted?