Surfer, it's a tough one.
I have known a few poly/mono couples that have been in the situation of the poly person waiting for the mono to come to their sense and be poly, and the mono person waiting for the poly person to come to their senses and "settle down" with them. What I have seen in the past has not led to a terribly healthy dynamic, with each side pushing, sometimes subtle, sometimes trying to "force the issue".
But I think the commitment you have to not just walk away and try to make something work is admirable.
I think that initially education is the key. There are lots of misconceptions and assumptions about polyamory out there - make sure that these have been fully researched before any decision is made.
Eventually, the process comes down to comparing your "bottom lines". Could you be happy in a monogamous relationship? Could she feel comfortable with one of the very varied forms of polyamory that there are out there?
If not, then the tough thing to realise is that you have "grown apart" and that there are some fundamental differences that are going to get in the way of your relationship together.
This is far from an easy process for both of you, and I send positive thoughts your way.
"Listen, or your tongue will make you deaf." - Native American Proverb