Just wanted to say you're not alone. There are several people on this forum who have gone through the exact same thing. I'm sure you'll hear from some of them soon.
In my opinion, S isn't being very respectful or thoughtful of you right now, and I can see why you feel his concern is more about your co-worker, than about you. It may be that he has constructed a fantasy in his mind that you will adjust easily to this betrayal of trust, and be happy to transition smoothly to poly. He certainly sounds impatient, and afraid of losing his new love. He needs to recognize how much love, attention and reassurance you need from him, if there is any chance of this working out.
You sound like you're in so much pain; my heart goes out to you. Hang in there!