I wonder if you can "diffuse" the obsessive excitement of NRE by spreading it toward Alex too. Like, if you find yourself checking obsessively for a text from K, could you text Alex to tell her you love her or to send her some random picture or whatever? Maybe that will help.
I also struggle to know what "falling in love" means and how that is different from new-great-sex-energy and from the "I want to be with you and only you forever" feeling that seems to be how most people experience love. I don't trust the former and I don't experience the latter, so...?
How did you respond to Alex's question about your feelings for K? How did she respond to your response? How would Alex feel if you really did fall in love with K?
Sounds like things are going really well. Keep at it.
Single, straight, female, solo, non-monogamous.