The whole falling for fuck-buddies does happen.
That's how Shannon and I got together. After we had both successfully had many fuck-buddy type relationships before. Although even then we still denied it for months (to the extent our mutual friends declared we were dating no matter our protests). Even then I didn't believe them I had "real" feelings for Shannon until a pile-up on the motorway we got tangled up in - I was putting it down to NRE, and all our mono friends just not understanding.
But, perhaps consider this a time to re-evaluate other relationships, of all sorts. Start pouring energy into them and see what happens. Re-direct some of the NRE into your long term partner, into others too. Shannon always knows when I have a new one, as the way I act changes, I become more attentive and loving, as I begin using the NRE in my other relationships.