I had an undiscovered affair with a work colleague towards the end of my 20 year marriage. I knew the marriage was over, I'd tried everything to make it work and knew I was going to leave, just a matter of when.
OM was also married (6 years) when we started the affair, but was in the process of separating. He was divorced a few months after our affair started. I felt bad about his wife, but justified it by telling myself that it wasn't me that was her problem, it was her husband that was her problem.
It was about a year before I actually left my husband. During that one year affair I didn't feel guilty at all. But that was because I had no respect left for my husband. I did hate the lying though, finding it exhausting sneaking around. (poor me huh?).
After I got divorced and processed all of the emotions around the end of the marriage, I did experience strong feelings of guilt about the affair for quite some time. I never told exH about the affair.
OM and I never got into a relationship but we continued seeing each other on and off for several years. There were feelings between us but the unspoken truth was that we would never, ever trust each other. And I suspect he was a total player anyway, I'm pretty sure I wasn't the only OW he'd had...
As for now, well I've been single for a long time, I hear from OM now and then, but haven't seen him for about 3 years. I still feel bad about cheating on my exH. I also think about OM's ex wife, and I hope she's now happy with a better man than him.
If I am ever in another relationship, I would never do it again.