one's own person

be one's own person (be independent, be responsible for oneself) — сам себе хозяин
Also, be one's own man/ woman

Example 1: We can't get Jerry to follow the rules — he's his own person. — Мы не можем заставить его следовать правилам - Джерри подчиняется сам себе, у него нет начальников, он сам себе хозяин.

Example 2: As you can guess Jerry's his own man since he earns his living and manages to support his old mother and unmarried sister.

Example 3: Young people may have viewpoints that are different from yours or may take up activities that you don’t understand. Try to see this as a good thing. They are learning to be their own person. (betterhealth.vic.gov.au)

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Chaucer used this idiom in Troilus and Cressida: "I am my own woman, well at ease."

See also
[law unto oneself]
call the shots / call the tune at
[call the shots]
[footloose and fancy-free]
[сам себе хозяин]
[твёрдо стоять на ногах]
[делать по-своему]