Around 9-10 years ago, before we were married, my husband carried on a sexting relationship with a woman he worked with. When I found the messages (I was suspicious owing to flirtatious messages they exchanged publicly on Facebook, so checked his phone), he was distraught and promised never to do it again. I gave him the benefit of the doubt and put it down to a middle-aged man being flattered by the attentions of a much younger woman.
Then, a few months later, I checked his messages again, just on the off-chance - let's call it 'housekeeping' - and was surprised to find more messages between them, all explicit in their nature.
Nothing in the messages indicated they had met up and actually made good on any of the content of the messages for real. We split up for a few weeks and I eventually decided to forgive, forget and move on.
But I didn't forget, and I'm not sure I've ever really forgiven him, either. I said all the things I wanted to say at the time and he definitely knew how hurt and disappointed I was that he'd broken the promise.
In retrospect, and possibly because I feel now that I've outgrown the relationship, but feel trapped and suffocated so haven't actually had the balls to leave yet, my mind keeps winding back to that betrayal, regardless of the fact he didn't actually 'do' anything physical with this woman. It's still betrayal and it feels just as real as if he HAD been intimate with her - for me, anyway.
OP, you need to be honest with yourself as to whether you can trust him - even if you still have doubts, which, let's be honest, most of us would. The fact they still work together compounds the issue, too.