Any help for internalised polyphobia?

TW: Some mention of polyphobic views that I don't believe in but that my brain tries to apply to me dating

For first time in my dateing history I have a chance to engage in non monogamy in safe condition with people I trust and that have experience.

I feel secure in myself in my mental health, my long term partner is also someone I seen act over and over in ways that make me feel safe (I would say they are my first truely healthy relationship), and same goes for the other potencial person I am thinking about getting into fwb with(they are part of happy long term polycule and have many fwb relationships that seem like they never emploded). My past poly experienced was ridden with a lot of abuse, and tho I can see that it wasn't due to poly somehow being evil but due to people that I was involved with and also me being in a vulnarable time in my life, it still left an anxiouse pit in my stomach conected to the fact that everytime I engaged in polyamory something terrrible then happened to me. Monogamy feels like ,,safer,, option.

I have this tought that my curent long term relationship is stable and fullfiling- why would I want to rock the bout in any way? I feel the internal judgment of ,, if you loved them enough you wouldn't WANT that,, or that its an immature whim.Wanting something doesn't feel like reason enough, and frankly I wish I didn't want it cause it feels inherently like a ,,bad thing,,. And on top of my internal anxiety of course society is incredebly biased against polyamory so that doesn't help.

I had this ,,want,, about that person for like 1 year now, few weeks ago expressed that to them cause it was getting pretty obviouse I like them (ofc all of that with knowledge of my partner) and they feel the same-recently they asked me if I want to explore thouse feelings into physical side of things. And I am scared to. I don't wanna fuck anything up for me, my partner, this person, their polycule we are also friends with.