It's so beautiful when we find a partner with whom we click. That NRE is the greatest drug in the world sometimes. J sounds great for you.
I have completely tossed aside my poly leanings and my definite non-monogamy over and over again because I really wanted to make a monogamous relationship work, or I felt I had to, or something. (see other threads, haha). So I would only caution you to just not throw the baby out with the bath water. Keep being honest with J. She knows with certainty she is mono. Do you know with certainty you are? I read your explanation above, but it's not clear you can really abandon yourself to her honestly.
I do agree you should follow your heart, totally. But the cautionary note would be to not let the NRE wipe out a commitment to honesty.