Op, I think guilt is entirely normal when you've treated an innocent person very badly, and shows that you are a decent empathic human being.
You say that his wife wasn't abusive or awful, yet he cheated on her several times before leaving her for you.
I can tell you that finding out that the person you loved more than anyone, that you shared a history and happy memories and children with, that you thought had your back, has lied to you every day whilst whispering sweet nothings to another woman is utterly and absolutely devastating.
I spent a long time feeling like I was going mad, knowing the temperature of my marriage had changed but not knowing why, being told I was imagining it, crying myself to sleep.
Several years have passed but I will never recover. I feel ugly and disgusting, stupid for believing him. I doubt my own judgment of people. I don't trust anyone. I certainly haven't dated. Counselling hasn't helped.
Reading posts on mn often upset me because ow often refers to the awful abusive ex, and I know he said those things about me and she believed him but they weren't true (he's since apologised). People also seem to react badly to the wife becoming angry or vengeful. I didn't but I can see how some women feel so terribly wronged that they lash out in anger, hurt and frustration.
And the most upsetting of all : we were soulmates, it just happened, we're very happy now and he'd never do it to me.
Because what does that say about his first wife, of years or decades, that he proposed to and had children with? That she was a mistake or a blip? Why was it ok to treat her like that, to humiliate her and lie to her? Why does your right to be soulmates trump her right to be treated with respect and kindness?
We're all just out here, pretending we're fine to minimise the humiliation, to protect our children and to avoid looking like the bitter stereotypical ex that bores people and disgusts people in equal measure.
Having said that, finding out ow felt guilty about it wouldn't help me really. Your guilt isn't helping anyone. I tend to think you should draw a line under it and let it go.