Finding oneself, or inventing oneself?
On the axis: Essence and existence · self-realization
The injunction "be yourself" is itself divided: is self-realization bringing to light a self already there, or freely inventing one? "Become who you are" tries to hold both at once.
Justification
"Become what you are": there is no fixed human essence to realize, but a task of self-creation. The subject, the soul, the substantial "I" are grammatical illusions; what we are is the ever-open work of self-fashioning, of giving style to one's character.
Become what you are.The Gay Science, §270; Ecce Homo, subtitle
The Gay Science, §290 · Ecce Homo, subtitle "How One Becomes What One Is"
Justification
For a manufactured object, the paper-knife, essence (the artisan's concept) precedes existence; tradition thought of man likewise, created according to a human nature conceived by God. Remove the creator, and the order reverses for man alone: he exists first, with no prior definition, and it is from being nothing at the outset that he draws the full responsibility of making himself.
Man first exists, encounters himself, surges up in the world, and defines himself afterward. If man as the existentialist conceives him is indefinable, it is because at first he is nothing. He will not be anything until later, and then he will be what he makes of himself.Existentialism Is a Humanism
Existentialism Is a Humanism (1946) · Being and Nothingness, Introduction and Part 1 (the for-itself)