at the risk of sounding too sensitive.....
your first question said a lot about you, and the answer is simple. yes, just like some people are built for it, others are not.
what i would say about your friends reactions.... you are not alone, poly is not for everyone, in fact... it is actually for very few of us. our best friends just shake their head and call us insane. most of our other friends just dont get it. they think we are crazy too. the way we handle it, is not to take it personally, we accept that very few people will get it, jealousy runs through our veins by nature and evolution. it helps if you understand why you can be poly, not why anybody can be poly, we are all different. for us the primary reason is we believe that love itself is boundless, we both hare a large capacity for love, caring and compassion. when you care that deeply about someone, well romantic attraction happens and runs hand in hand. ha ha, my friends still dont get it. i wont go into the whole philosophy we have that usually gets them to understand the concept, though they could never do it themselves.
the above statement also hints at what you might be going through with your romantic interest. unfortunately you have been drawn romantically to an attached monogamous person, who will more than likely be completely unavailable, and cause additional drama, and heartache for you or someone else. i know, its messy and not easy. the other clue is your romantic withdraw from your boyfriend, which can be common during nre, or for people who are wired for mono.
you my dear need to do a bit of soul searching, self reflection, and see if poly is really right for you, or even what version of poly is for you. oh yes, everyone has their own method and madness, boundaries, etc....
my intent is not to sway you one way or the other, i wish you the best in your romantic future.