leg up on other critics:
1. We all pay attention to ourselves and our own work *much* more than others do.
2. We see everything we do. There is no curation or putting the best foot forward. We are fully aware of both feet.
3. Nothing we do amazes us because we are used to it. Like, there is no talent I have where I think, "wow, I can't imagine have that level of talent." Everything I do feels like it could be done by any old idiot and for everything I am good at, I have a point of comparison for someone who is way better. In fact, part of how you get good is that you are able to identify your own flaws.

Anyway, I think "I am my own worst critic" is a line that could be used disingenuously by a narcissist, but unless you are truly asleep at the switch *and* have lots of hostile commentators watching your every move, I think many, *many* people actually are their own worst critics.