OP - there's a lot to be learned from this, including what you want from a relationship. Look at it this way, you had an exit affair, your marriage was effectively over and you needed a reason to end it. That's your excuse, the affair made you 'brave enough to admit defeat' on your marriage, it's very hard to walk away from a relationship that's not working when there's children involved if you don't have a solid reason to do so. The OM gave you a solid reason.
so that's what you were doing, what was the OM doing? His marriage isn't failing, he wasn't looking for a way out, he wasn't deeply in love with you (as he wouldn't be happy for your relationship to end if he was), so what was he looking for? A shag? An ego boost? Just wanting to 'play' a bit and have an intersting distraction from work?
A person who's had an affair for the reasons you had it could be trusted again, because it's unlikely the same situation would present itself again, and actually, you'd be more likely to just end the marriage rather than look for an exit once you'd realised it was dead. Whereas, the other man's reasons for having an affair seem to make him the sort of person you can't trust. Nice to have fun with, nice to be friends with, but to share your life with? God no.
As for getting over it, you need to start putting feelers out for a new job, it might take another year or two to find that job, but you might be surprised. At least then you could feel like you've made the emotional break.
Also, be certain, people in the office will know about your affair. As sexist as it is, woman who have work-place affairs are usually considered far worse than men, long term it will have negatively effected your career in that company. You might never be told about it, it might not be obvious, but you are going to have to work 10 times as hard as colleagues to get promotions/recognition. You will have to prove yourself despite your 'reputation'. Sadly, he won't face that in the same way. So long term, you should leave your current employer. You have already 'lost' any status and success you have built up there. You just don't know it yet, but slowly, it'll become clear.