relax, you're way over thinking this. Being "free" is to me what being poly and open are all about. Find joy in your partner's joy (the term is compersion. And in "my girlfriend just had an orgasm, and i'm feeling very compersionate twards her right now). It can be a rocky road, no question. But the rewards are very often well worth the uphill climb.
remember, just because you feel fulfilled by one, doesn't mean you can't love another.