I meant to say earlier, I was glad to hear you turned down Sam's offer of hooking up. Seems like the right decision.
As for the other stuff...
Your questions always resonate with me...because I don't know the answers either.
I too struggle with the feeling that other people are able to be with someone, to feel purely happy and to know that they want to spend their life with that person...and moreover, that poly people can have that feeling for more than one person.
But I don't experience that feeling. I've figured out that I'm happiest, most in tune with myself, most sure of myself, most content with my life, and most clear about my future path, when I'm single.
I don't want to be celibate or totally alone, and I don't want to not care about other people and their feelings, so I'm slowing figuring out alternative forms of dating/sex/friendships that work for me.
Unlike you, I'm not living with a partner who loves me desperately, so I never had to make the choice to give that up.
Like you, I spent a long time in a situation where I was involved with two people of totally contrasting personality & relationship types. One was a boyfriend I cared deeply for, considered my best friend, and would have moved mountains before hurting. We had a million things in common, including professional and social spheres. But we also had poor sexual chemistry, and he had his own sexual issues which I thought he needed to explore on his own.
The other guy was just pure sexual chemistry, not at all "relationship material," hardly had anything in common with me. He made me feel amazing and I liked him immensely. He was also a complete moron who was stoned a lot.
Neither guy was right for me, but my instinct to be single was.