Just catching up on your story. I'm always amazed when someone is given the, "could I have a second chance?", question and turns it down flat. It's not an indictment of you; it's just foreign to me. But what else can you say when your STBXW has vacillated before and is presently living with some other guy? Not much to say except no. It hurts a bit I'd guess to have to say that.
Glad your life is otherwise well adjusted. Nice to hear. I'm glad you got the chance to discover that life doesn't have to end on account of all of this.
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer