“To be nobody but yourself in a world which is doing its best, night and day, to make you everybody else means to fight the hardest battle which any human being can fight; and never stop fighting.”
Or the short version:
“It takes courage to grow up and become who you really are.”
Both, e. e. cummings.
Or the one I like best:
"Be yourself, everyone else is already taken."
"Sometimes I pretend to be normal, but it's boring and I go back to being me."
"You might as well be yourself, people won’t like you anyway."
"Tres verbo dictom"