My DH also had a ONS with a woman from work, OP, so I truly feel your pain and understand what you are going through. It is the worst thing that has ever happened to me, worse than any bereavement I've gone through, because it's so much more than a loss - it's betrayal and the tainting of all your happy memories together, the pain of imagining what he did, the humiliation. And the OW had gone around their place of work bragging about it to anybody who would listen, which even now fills me with rage. It is absolutely brutal. Why did he do it? Why does anyone. Because somebody showed an interest in him and it boosted his ego, because our DD had anxiety and was constantly sleeping in our bed so there wasn't much chance to be intimate (not my fault), because his mum had recently died and he was looking for a distraction from his grief, because I wasn't new anymore, who knows. Probably all of those things.
I found out about 6 months after the event. DH and I are still together. I am still suffering BUT I have noticed that it is improving gradually with time, it's just very slow. I no longer think about the act itself, though for the first 8-9 months it was like a video loop in my head, total torture. I think less often about how much I hate the OW, whereas before I would imagine pushing her off a cliff (and yes I know how MN always says it's the man's fault not the OW, and yes it is his fault, but the OW is also complicit, just less so - and my feelings are valid, they are what they are and I can't help them) - but this is partly because of the fact she bragged about it to people, it seemed so brazen and cruel, like a violation aimed at me. My feelings have evolved over time and continue to evolve, so my hope is that over time it will fade more. I think there was an element of DH taking me for granted before, but this has never happened since, I have clearly remained his top priority ever since and he is very very keyed in to my feelings, and always making sure I'm okay.
The truth is that I will never probably get over it, however, even if I moved on and met somebody new I would still now sleep with one eye open, because I have been cheated on 4 times by different partners, and I feel very damaged emotionally, it has changed me. And the truth is I am more afraid of being cheated on than I am of dying - dying would be easier. DH and I get on extremely well, we are a really good team, and I know he has learned a hell of a lot from this horrendous experience - enough I hope to never do it again. I would love to say that one day I will be able to relax enough to never think of it and to trust 100%, to have the comfort again of actually having peace of mind, but I know I will never be completely happy again because this has happened. I know that there are many people who cheat and who realise (too late) what a terrible thing it is and know in their hearts they will never do it again, but there is always somebody on the receiving end who is the collateral damage that happens while they go through this process of growing up and the penny dropping that it's not worth it. That's the sad thing, there are people who, having cheated and learned from it, know they will never do it again, but they have already caused so much damage by then.
My DH understands, as I have said very plainly, that if he stays faithful to me I will stay with him forever and love him faithfully and look after him when he's old and we, and our DD, will have a lovely, happy life together - I will stand by him through thick and thin. But if he sends so much as one inappropriate message to another woman I'm gone. There will be no more chances. If that ever happened I would be screwed financially as I only earn 16k a year and that is full time already, but I would still walk away because he'd already have had and wasted his last chance. I hope that never happens.
I know there are people who would never cheat - I know this because I'm one of them. I wish I had married one, but you don't know until you know.