OP I'm posting this in order to support you and to help you move on, but I think what's holding you back is that you might have a flawed script operating about why the OM stayed in his marriage. On a professional basis, I've spoken to lots of men in his situation and they tell me something that they never seem to have the courage to tell the OW - that the main reason they are staying is because they love their wives more and that the loss of her would therefore be far more painful than the loss of the OW.
I've also spoken to lots of women in your situation and this is the bit that they struggle with most. It's especially difficult for women like you who are doing all the right things i.e. not hating the OM, taking responsibility for their actions and trying to get over an enormous loss. But the thing that holds them back all the time is this belief that if it hadn't been for the kids, the house and the finances, the OM would have left his wife.
When OM aren't honest about this when the relationship ends, it is an unfairness to both of the women in the triangle - and all because he cannot bear to appear the 'bad guy' to the OW who might then think badly of him. But by lying about his real reasons for choosing his wife over the OW, he is committing a further offence against his marriage by letting the OW believe that if all things had been equal, she would have been his choice - and a further offence against the OW by letting her believe that he is making a noble sacrifice and giving up the true love of his life.
Something I often suggest to both OM and OW is that they find a way to permit that message being given; sometimes in the form of a letter if personal contact would be too painful for all 3 people in the triangle, or a final 'clearing of the air' meeting - but this can be very difficult for the partner who is still trying to regain trust.
To women in your situation, if that opportunity is not going to be available, or if it does and they still hear the same 'if it wasn't for the kids.....' message, I encourage them to look at things a bit more logically. At one end of the spectrum is a man who made the very human mistake like her of falling in love, while still loving his wife. It is of course possible to have very strong feelings for two people. At the other is the man who does this all the time. Then there are huge shades of grey in between; people who were merely infatuated but said it was love, people for whom the affair was more about the feelings being reflected about him and less to do with feelings about the OW. But invariably, if after being found out he stays with his wife and wants her forgiveness, it is because of his feelings for her and what he gets from his relationship with her that compels him to make that choice.
Of course like his wife, it's often very difficult to separate out those feelings of love from the fear of upheaval that a divorce will herald and also the feelings about the impact on the children - and on both of the couple if they have to parent separately. But for a couple trying to rebuild a relationship after an affair has been discovered, it's something I encourage them to do, because if they are staying together because of those other ties that bind and not because of their depth of feeling for one another, it will not work. IME, the marriages that survive affairs are the ones where the love is still very strong and where both parties are very clear about why they are fighting to stay in their marriages.
That's how these admissions from OM emerge - even in the safety of a one-to-one without their wives being present. When push came to shove, even if the feelings for the OW were strong - they weren't strong enough, or as strong as they were for his wife.
Practically speaking, it isn't going to get any easier while you're still working with him - and I agree with an earlier insightful post that there is a trade-off for you of prolonging this agony by continuing to be co-workers. Try to work towards that trade-off being of less importance or necessity to you.
Do consider some therapy of your own to help you process some of this. You made a human mistake and you need to forgive yourself for it, but I think the key to this is forgiving yourself and him for respectively believing - and giving - a false script. His motive might have been both not to hurt you unnecessarily which he thought was for the best, but also quite selfish in not wanting you to think less of him. If you can get some honesty, consider it. If not, apply some logic to the situation and then let it - and him - go.