Is it all just insecurity?
No. Some of it is preferences and resources. It is not all about "insecurity."
I feel wired for poly. Had it for a while. Then didn't for a while. Why? It isn't because my capacity to love others went away. I've been crushing on people all along! But I did not act on seeking new partners and still do not because my resources are tapped right now. I'm good where I'm at.
Various things affect willingness and dating life besides inner emotional state (ex: insecure, self esteeem, etc) or personal preference/wiring (monoamorous, polyamorous, something else).
Non human resources -- money access (takes some to date!), material goods access (car? bus? bike?), community resources access (where I live can affect dating ability - where one can go or do on a date)
Human resources -- time, energy, attitude, leadership, communication / conflict resolution skills all go in that bucket.
Everyone has a different saturation point and there is nothing wrong with keeping it at 1 (just you ) or 2 (you and one partner) if that is where it is for you.
There is nothing wrong with monoamory. There is nothing wrong with polyamory.
But I would not go off to any new partnered things without making time to assess myself for readiness -- and that could include working on getting secure if "insecurity" was a problem for me. If already partnered, getting a sense of how welcoming they'd be of a change in the configuration because changes in my life affect them too and things may need renegotiation.
Don't know if that helps.