Thanks everyone for your input!

I went to see my therapist yesterday and had quite a profound discussion. Turns out my irrational fear comes down to fear of abandonment, rooted in neglect in childhood.

This is how it played out in my mind (irrational, I know): I saw her hanging out with her ex again, making a poor decision to get back together with him, and me having to try to remind her of how she herself said it wouldn't be good for her to be with him.

This is what my therapist calls the "rescue position" - a deep-rooted habitual position I assumed from a very young age because, even before I had words, I perceived that my parents were unable to make good choices for themselves and were unable to truly see me and perceive my (valid) childhood needs, let alone meet my needs. I took up this position to, in a very child-like way, show them that I am actually good enough to be worthy of being seen. (When you're very young, below the age of 5, you have a built-in idealisation mechanism that says to you that there can't possibly be anything wrong with your parents, so you have a tendency to blame yourself for your needs not being met).

Funnily enough, it was this exact dynamic that played out over and over again in my relationship with the narcissist, where, in reality, she was unable to make good decisions for herself, because she was so disconnected from herself. I was drawn to this destructive dynamic like a moth to a flame because it reminded me, on a very deep level, of my primary "loves" - my early family relationship dynamic. I constantly felt like I was trying to save her from herself, and convince her of my value.

Being reminded of this dynamic, due to my imagined scenario of my current girlfriend visiting her ex and how it would play out, kicked my anxiety into high gear.

The reality, and what I have to remind myself of, is if she had to go back to her ex, it wouldn't reflect on me or my character or my actual value in any way. It would imply that we're not right for each other, and in the worst case (if she had to go back to him knowing that it's not good for her), a character flaw in her, which isn't desirable in a partner anyways. In any event, assuming that "rescue" position isn't healthy, and nobody should ever have to feel like that in a healthy, loving relationship where both parties are making good choices for themselves and each other.

This was all, of course, irrational fear, and I have to work on not allowing my mind to jump 50 steps ahead and live in my imagined future (easier said than done, naturally). I do love her, and wouldn't want to restrict her from experiences that're good for her, and wouldn't want to be restricted myself either.

Man, just thinking about this now, parenting is such a massive responsibility that requires a significant level of emotional maturity. I don't think many people realise that.