I've only had time to read the original post but I thought I'd put in my two penneth in case it helps.
My dh had an affair, also with a workmate, ten years ago. I know how it hurts. It was a very difficult time for us both and it really took some getting over.
I stayed with him because I believed it's what we both wanted. He managed to convince me it was what he wanted and that he'd never stopped loving me. (That can be hard to believe at the worst times)>
What helped me (but it's not for everyone) was making sure I knew everything. We took time apart writing down our feelings and facts and events then spent the night in a hotel together taking turns to read what we'd written, uniterupted. He told me everything and some of it really hurt so badly. BUT I think this night saved our marriage.
I believe I was far more fearful about what I didn't know. When I felt I knew it all (even the really hurtful bits) I felt I could move on.
It took time but I genuinely feel no pain surrounding the subject now and I think this is why. Laying the facts bare was like digging oput poison from a wound. It hurt but it meant that real healing could occur after it was done.
I also accepted some responsibility for what had happened. (I was ignoring him, taking him for granted, focussed on other things etc etc.) That helped because I felt it wasn't totally random and outside my control.
I wish you all the best with this.