No particular moment sticks out. I have been like this for as long as I can remember.
If you want to know when things were solidified, however, I have two. The first was when I was "nearly kidnapped" as a kid. It wasn't much, some guys in a van asked me if I wanted some money, I said no and ran off, nothing else came of it. However, I was told at an early age about pedophiles by my parents, and learned about them via the news. I didn't get the "don't talk to strangers" treatment, I knew more or less what would happen if I didn't watch myself. However, until that moment these were all just things that happened to other people. You may be aware that this could happen to you, but you still don't see it coming when someone tries it. Even though I got away, I got home unharmed and never saw those people again, it really put things into perspective. As a result, my already slightly existing paranoia over pedophiles increased tenfold. Now it wasn't just a "be careful or this might happen!", this was a concrete threat. I wonder, if that hadn't happened, maybe the symptoms of mental illness that were already around would have been much more mild and could have been treated. Maybe this was the beginning of the end, the start of my journey towards being unfixable.
The other was when I was arrested. That eliminated all possible hope of me having any future worth living for, as well as the beginning of an incredibly traumatic experience with the police that lasted three whole years of my life. If it wasn't already, then that certainly sealed my fate.