Characters
The worst feeling is suddenly realizing that you don't measure up. And that, in the past, when you thought you did, you were a fool.
— Angela • My So-Called Life

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker describes the most painful sensation as a sudden recognition of falling short. It also poisons the past, because earlier confidence now looks foolish. The sting comes twice: the present shortfall, and the retroactive embarrassment of having believed otherwise.
Context reviewed