A warm welcome and an invitation to introduce yourself!

It's the self fulfilling prophecy, alright. What is driving my insecurities is that sense of shame/not being good enough that I inherited from my childhood, like many people here (at least APs). But then these insecurities do indeed become reinforced by other people who tell me that I am insecure. And since I believe them, then I continue acting in an insecure way.

I see where you are going about letting insecurity define you. If you let it, then it will for sure. It's precisely that distinction that I struggle making: that insecurity is not me. But since insecurity overshadows everything, this is what makes it so challenging. It affects everything. It can even turn positive qualities into something we play down. In the end, it is no longer possible to tell the insecure you from the real you. I like that this distinction seems to address many problems at once, as it comes quite close to the root of these problems.

Good question regarding whether insecurity is a flaw, given that everyone experiences at one degree or another.
The only thing I know is that it is considered by many as unattractive, especially in romantic relationships. And since I let define insecurity define me, it affects the whole me. The whole me is therefore unattractive, instead of the insecurity only.