As another poster said, his feelings towards her haven't changed just because you now know about her.
Whatever it was that led him to start an EA (maybe more) with her is still present.
Their affair hasn't come to an end because they wanted it to, but only because you've told him to end it.
I don't believe for a minute that things are going to end here. Him working in such close contact with her every day makes it near impossible for them to just switch off their feelings. Maybe you knowing just makes it even more dangerous and enticing.
Your DH knows he's got away with it (for want of a better word) as you haven't left him or asked him to leave, so what's to really deter him from carrying on but just making sure he covers his tracks better?
With the best will in the world there is no way you can monitor what they get up to at work. You said that if he contacts her without you knowing you will leave him, but how will you know if he does?
They will have umpteen opportunities every day to speak to each other, in the corridors, on the phone, via work email accounts and their proximity every day will make it impossible for you to monitor their contact.
You already know he is a man you can't trust, a man who is prepared to lie to you and deceive you and one who has very recently purposefully engineered a situation so he could be alone with this woman in the knowledge that something would happen.
Can you really trust that in the 10 hours a day they work together he is going to stay away from her?
I think it's wonderful you've found your inner self worth and you've found your voice but please be careful and try and protect yourself from being hurt further by this man.