A few conclusions
Thanks everyone, I did need the reassurance.
The "lessons" tag did help. I will follow up on others, too. (Had already read the Second's bill of rights, and many other forum posts!) Thanks, Clairgoad, RedPepper, TruckerPete, Ariakas, Tonberry, and Nycindie!
I am very independent, truly. My life is so fun! I absolutely did not give any signs of jealousy or cling or anything, and don't require a partner for emotional stability or happiness - I was merely very fun company, and a great listener. It turns out the problem was him. I have concluded that his NRE wore off already. (I just read that term today.) That's hard for me to admit.
We had a pretty fair talk this weekend, and it had the effect of 1) making me appreciate the honesty involved in polygamous relationships; I've never met a man who can talk about his feelings like that. Staggering. And 2) making me realize my intuition was right. THAT is why my insecurities were coming out. He says "I think you like me more than I like you" and he doesn't want me to get attached. So I guess this has nothing to do with polyamory, and again to do with me choosing the wrong partners. He doesn't want it to become any sort of relationship.
And yet still wants to hang out, and sleep together? My next intuition is that I should avoid the sex part, at least. I just hope that the advice about meeting new people is true, and that we can still have some adventures.