Hi, Bluefairy, welcome to SI. So sorry you had to find us.
Dealing with betrayal is probably one of the hardest things you will ever go through. The man you trusted has broken your trust in the most despicable way. Please find a good trauma therapist for yourself to help you process this nightmare.
Understand cheaters lie, 99.999% of them. They minimize their actions, gaslight, blame others, it's so predictable.
My husband had an affair with a work colleague who lived and worked on the opposite coast. He would have to visit her site several times a year. There was no way in hell I would ever accept him coming in contact with her, even through company business. He dropped her like a hot potato and immediately started looking for another job. He understood I wasn't going to tolerate one more minute of disrespect as far as the other woman was concerned. It took him several months, he found another job, actually a better job.
If your husband and his affair partner work together, I'd question if it was just emotional. Too many of us here, including myself, were told they just kissed, blah, blah, blah only to find out later that it was much much more. Ask him if he's willing to take a polygraph test.
If he truly wants to save your marriage, he will do everything necessary to help you heal, finding a new job, being honest, accountable for his time, access to his phone, emails, social media, etc. Sometimes cheaters take their affair underground, just be vigilant.
He can never have ANY contact with her again. Ever. No compromising. It sounds harsh, it is, he made his bed, he has to face the consequences.
BTW, is the affair partner married?
In the meantime, take care of yourself, eat, stay hydrated, and meet with your doctor if you are having coping/sleeping. Many of us here have taken medications temporarily to help us through those rough few months.