Self-Doubt
Less a personality trait, more a voice with a specific job.
The inner critic is the internal voice that narrates your mistakes, second-guesses your decisions, and sets a bar that's rarely satisfied even when you clear it. Most people experience it as just their own thinking, which makes it hard to question.
This voice is usually learned, often from an early caregiver, teacher, or environment where criticism was more available than encouragement. It develops as a kind of preemptive defense, criticizing yourself before someone else can, or pushing hard enough that mistakes become less likely.
The inner critic usually isn't interested in accuracy, it's interested in safety. Telling it to be quieter rarely works, because from its perspective, staying critical is what's been keeping you safe. Therapy tends to focus on understanding what the critic is protecting against, rather than trying to argue it out of existence.