"What do you want to see when you look in the mirror?
Right now you have options. You have power. You have control. You have self esteem (you do, despite what he's done). Let him wander in and out of your life at his will and you lose all those. Do you want to see that woman every day?"
This.
It's so hard for you, OP - so heartbreaking, particularly when you thought you had settled into a committed relationship with a decent man.
But as you know, if he were as committed as you, he wouldn't be investigating what might happen with this woman (a work colleague, no less). And like PP, I somehow doubt he has told you the full story on how her confession came about. Perhaps she is as brazen as this approach suggests but given that they work together, which brings all kinds of other factors into play, I would imagine she was at least given a fair amount of encouragement first.
If this had happened in the very early months of your relationship when you're both still feeling your way into it and working out whether or not there's a future, I think it could be surmountable (I did something a bit similar to my now DH), but by 18 months in, there shouldn't have been a question.
You've said you know what you need to do but you're not sure you have the strength to do it. Could you do it in stages?
Perhaps for now you could say that you need space to reflect on whether this is still the right relationship for you because his actions have told you that you and he are not coming from quite the same place. There doesn't have to be blame. You can just be calmly matter of fact about it. And then in a week or two's time, you can say that having reflected you don't see a future because you need someone you can trust to remain committed and he has shown you that he is not that man.
If giving him space pushes him into her arms, well the chances are that he would always have ended up there. And if he can do this once, he can do it repeatedly. And the impact on your self-esteem will be enormous.
However, the impact on your self-esteem of saying you love and value yourself enough that you're not going to be someone's Plan B - I think that will actually be very positive.