"this should be the most fulfilling situation in the world for each of us"
The important thing here, I believe, is that you accept that what you feel "should be" is not necessarily what other people want or can even understand. This statement is from your perspective.
Have you thought about why it should be the most fulfilling situation? What would be the difference between living in a loving relationship with your wife and friend without the aspects that make it polyamorous between all three of you?
What is in it for each individual? You, your wife and your other lover. You can even go as far as to write it down individually and then compare..see how close they are. Just a thought.