Romantic Love - Yeah, it comes and goes, especially when things seem to be changing a lot.
LB is 10 - Our babies don't need quite so much of our attention anymore. They are much more independent and it can be exciting and depressing at the same time (my oldest will be 18 in 2 weeks and off to the Army in August). They still need us, but they don't want us hovering (or they don't want it to look like they need us). Learning how to shift gears to the more independent child can be difficult and disconcerting.
Age - we are the same age and at some point after 40 I started to wonder where I fit, I still feel like that sometimes. I watched a video on drowning recently and I realized, that's what I've felt like for a while. It may or may not be directly related to our kids getting older or maybe that just adds another complication.
It doesn't help when our loves seem to develop all these outside interests that take so much of their time/attention all about the same time.
You have spent years focusing on poly, relationships, etc. maybe it's time to just do stuff that's fun without any relationship expectations. Get back to your Art, take a class, teach a class, etc.