One observation: if you need more reassurance from her (instead of having to be the one giving it all the time), why not ask? As for "falling into polyamory," I think that happens to many of us. We often don't plan to become polyamorous, we just find ourselves in love with someone already partnered and look for a better way to handle it than cheating.
I guess I was lucky in that my "fall into poly" proceeded changes in my general ideologies that allowed for much more liberal ways of people relating to each other. The hardest thing for me was waiting for permission from the husband (whose wife I had fallen in love with). I guess sometimes it just takes time and patience to revise one's monogamous mindset.
Sorry, nothing earthshaking here I know, just thought I'd give what observations I had. I hope you and your partner can work things out, and she can be more at peace with the idea of being polyamorous.
Love means never having to say, "Put down that meat cleaver!"