Hi Sky Guy
I checked your profile and you're quite young so presumably your girlfriend is as well. I might be way off track and if I am I'm sure I'll hear about it, but I think young women as a group would be the hardest to interest in poly relationships.
I'm sticking my neck way out here and this is just my opinion.
You say she is the girl of your dreams so she is probably petty cool. Why would she want to settle for something that is very difficult for a person who is not poly to accept? What would be in it for her? She probably wants to look at securing a solid relationship with someone who has the potential of providing for children etc. A poly lifestyle would seem very strange when the cultural dream for most young women is still the lovely wedding and happily ever after scenario with Mr Right.
Again I may be totally wrong but I think people come to poly after quite a lot of living and the realisation that there are other ways to be happy.
It sounds to me as if you are just not ready to settle down yet. If you decide to give poly a go lay down your intentions from the outset.