A legend in (one's) own mind humorous noun phrase ironic
Used to indicate a person who overestimates their presence or importance when in reality it is not the case
OK, you can think you're a legend in your own mind, but we don't.
They behaved as if they did everything in this project. They are legends in their own minds.
She is the type of girl who is a legend in her own mind.
This expression acts as a noun phrase in the sentence. "A legend in (one's) own mind" is an ironic or hilarious way of the phrase "a legend in (one's) own lifetime"
Small and steady efforts can bring about big results.
Little strokes fell great oaks, I've finally accomplished that project.