I hate the cliche “be yourself.” It’s terrible advice. Instead, be your true self, which is the self that God wants you to be. There can be no true self apart from the Being who created us. We can never be happy if we create a self in opposition to our Creator.
How can we achieve this true self? Practically speaking, by pretending, as C. S. Lewis advises us: “Very often the only way to get a quality in reality is to start behaving as if you had it already.”
We tell children to act a certain way in order to form good habits in them. When we tell ourselves to act in a certain way, we exercise self-discipline in order to form ourselves in good habits, in virtue. We become what we pretend to be. The intention is not to deceive others but to change ourselves into our true selves.
Perhaps some saints aside, I don’t think we ever become our true self perfectly until we are in total communion with God after death. Life is a process of more closely discovering and putting on our true selves in Jesus Christ.