Step back. Take a breath.
You have a relationship now. Let it grow into the relationship it wants to be. Try not to get too attached to a specific outcome for this. Each choice you make should be based on the relationship here and now.
I've been mountain climbing and one thing that holds true is that you can't decide your next step based upon what's at the top of the mountain. You can only decide your next step based upon what's under your feet.
Grow and nurture the relationship you have now. Then see where it goes. Take any and all chances to be honest and open. For people who may not be familiar with it, it's a lot easier to learn that someone is poly without the added pressure of trying to accept it in a romantic relationship context. But don't force it into a desired outcome. Relationships and feelings rarely follow directions.