Long distance relationships suck. It's easy to fall out of love with a partner who isn't around, and into love with people who are closer. That can happen whether you've decided to be open or not.
I'm not sure if there is an answer to your question to whether some people are wired for poly or not. Lots of people have theories about this, but I don't think anyone can answer it definitively. I think that it is possible for just about anyone to love more than one person (we know this from loving more than one child, or more than one friend), but that doesn't mean that your particular relationship will work out.