TL;DR: Yes, I do, and I'll never forgive myself, even if I've been forgiven. It's what pushes me to do better and be better every day.
I thought I'd never cheat - all it took was having a severe postpartum depression with memory blackouts, paranoia, confusion (how do socks work?), panic attacks triggered by watching a commercial (the camera angle was ominous) and believing my (now ex) boyfriend was trying to kill me (he wasn't).
Do I feel bad? Five years later, you betcha that I still start sobbing about when I remember hurting someone that way (whether it's in the shower, in the car, in the grocery store).
My gathered wisdom from all of that is: birth control can fail, you don't have to always be genetically predisposed to PPD and PPP, and cheating is a very easy thing to do; You know how people say 'it just happened?' What happened was the first steps were innocent - you were being friendly or you didn't want to be rude; then the next steps you started lying to yourself 'What's wrong with having friends?', and then the path became more slippery. This is going to sound very cynical, but now whenever there is anything involving someone from the opposite sex I tell myself 'this could become an opportunity to cheat, acknowledge it and move on'. Weirdly enough, it's a very sobering thought, even if I trust my male friends to not be inappropriate and to respect my partner. Sure, someone might argue that it's easy not to be an absolute asshole, but I'd say it takes work not to be careless and thoughtless with people, and I'd gladly work hard every day of my life never to hurt anyone ever again.