I agree with Derby on taking it slow, but I can't help thinking that perhaps you are not poly, you just need another girlfriend that is more compatible.
I had a wife once that I loved dearly but, after 5 years, we just had different ideas about what our path was together and for ourselves. Perhaps deciding what goals you have would help you decide where you are going in your life together. If there is more than the poly talk that she is not too thrilled with then perhaps its time to think about moving on. On the other hand, if you find that in talking to her there is tons you still both want to do together and achieve then I think it will just take time and a whole lot of exploring together what this poly thing is about... be very clear though that poly is not just about sex. It is about relationships like the one you already have... not only will you be gaining another sexual partner, but another girlfriend... are you ready for that? Is she ready of that? Otherwise having an open relationship might work better. Or doing some swinging.
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