Oddly Wired
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competence

You see yourself as less capable than you are.

When tasks are difficult, people consistently exhibit a worse-than-average effect, underestimating their relative performance.

While people often report being above average at simple tasks, research shows this flips during hard tasks. Participants assume that if a task is difficult for them, it must be near-impossible for everyone else. They consequently rank their own competency below the actual group mean.

Your frustration with a difficult project is likely a reflection of the task's complexity, not your lack of talent.

Where this comes from

Study
Justin Kruger, 1999, Journal of Personality and Social Psychology
What the check found
The paper 'Unskilled and Unaware of It' (1999) does address this effect, though it is often conflated with the main finding regarding low-ability performers.
Confidence
Solid, with limits

Related

One of these every day, and every one names the study it came from. When a finding is contested, it says so.