I was the OW for a few months many years ago. The guy had a fiancee. No kids. I was single. We met at uni while doing the same course.
How I slept at night? At the time I was pretty bitter and jaded tbh. I'd come out of a really, really bad breakup and not long watched a parent drink themselves to death, plus was dealing with serious debt issues and health problems. The breakup and parental death broke me and I changed quite a lot. Felt quite angry at the fact I'd always tried to be a good, nice, decent person, treat others well, and that life had shit on me so intensely anyway. I snapped and started thinking 'fuck everyone else', I lost so much empathy for others. So I compartmentalised basically.
He kept telling me he was going to tell her, leave her, and live with me, and I believed him, felt like it was just another couple of weeks cos this or that kept coming up to prevent him from actually doing it... until after 4-5m I realised it was never happening and ended it myself. He was telling me he didn't share a bed with her or have sex with her and in a twisted way I felt like we had the 'real' relationships while they were just roommates. I even managed to convince myself it was for the best for all of us, including her, because it was better for him to leave her and she'd be better off without a cheat, than for him to stay with her living a lie while in love with someone else. How delusional I realise it is looking back.
There was also the factor of the intense connection and chemistry between us, I mean, when we were together there wasn't really much space for thought of her, we were so intensely into one another. The pure attraction and sexual chemistry was intense enough to blow away all thoughts of what I was doing to her. I don't buy that I was innocent and it was all on him because he was the one in a relationship, I was equally as terrible a person, I helped to do a horrific thing to another human who'd never done anything to hurt me. It's one of my only regrets in life. I really, really, really fucked up. I've never cheated on anyone in my life but realise I have no moral highground due to what I did with him. I've never been involved in infidelity before or since and believe I never would. It opened my eyes to a really awful dark side of me that I never want to revisit.
I'm utterly ashamed. They ended up marrying but then divorcing quickly after, I don't know why. He had cheated on everyone he'd ever been with. He told me years later she knew as he told her after things ended, though I don't know if that's true. We managed to stay friendly after it ended as we had to see one another for the rest of the degree. I don't really have any feelings for him either way anymore. I will always feel like a piece of shit for it.