Sounds like you and I are feeling some of the same things. Everything is good when your partner says, "Sure! Date someone!" But when they do themselves or in your case just mention it, it feels like the world just turned upside down.
I'm only about a week into dealing with the idea myself, and my E has only had one make out session with someone. Ive had horrible dark days, great days, and normal days. It's getting better all the time, but I'm not out of the woods.
Hang in there, communicate a lot. You'll get there.