I relate to this a lot.
For me, it's mostly that I am very skeptical of overly nice/happy people. Though I'm not so accurate and I get false positives, as I lean towards distrust.
I'm overly sensitive to changes in tone or volume when talking to people. It takes me a long time (usually at least a month or two) to start to be comfortable around a person. In this time of discomfort I struggle to open up, so when I do become comfortable my reservedness is already the norm :/
I'm definitely a pessimist as well, though I've been working on being less so. I've always seen it as a good thing too, something that protects me from further harm. I'm still grapling with the concept myself, and have a hard time explaining, but being more of an optimist can be freeing/relieving. (Particularly, I always want to expect the worst in every scenario and feed my anxiety in doing so. Where I'm learning to allow myself to expect a better outcome.) -- This is more around situations where the outcome is likely good, not expecting good things when it's more likely bad.
To be clear, for most of my life I could think of literally anything as turning out bad. Basically not expecting everything to have a negative outcome, which may not be your experience.
I'm hoping to be more comfortable opening up to people, as I've lost out on many opportunities to make new friends or be closer to the ones I've had. Growing up, I believed that as you become closer to someone you were bound to start fighting. Hearing my therapist tell me, "It's possible to have a relationship where you don't yell at each other" meant and still means a lot to me. I know now that I have the capacity (and support if needed) to leave any relationship that makes me uncomfortable. Still, easier said than done

Anyway, I think sensing narcissists, or even generally dysfunctional people, is a useful skill to have. I wonder if you are in a similar boat as me in struggling to form/maintain relationships? Do you feel like you miss out on what could be good relationships (friends/coworkers/...)?
@madeline7
I think I am now seen as overly sensitive, but my now much smaller circle of friends and family gets me, and are supportive.
This stood out to me, because after going NC I've been told that I'm overly sensitive. I think it's because I'm finally allowing myself to feel my own emotions and say 'no' to things for reasons I used to find weak (ex: canceling plans because I feel sick).