When most people say "Be yourself," they unconsciously mean "Don't be afraid to repeat and project your past experiences into the present." By "self," they refer to all the patterns of thoughts, feelings, and behaviors they have unconsciously learned and adopted in the past.
What else can be expressed spontaneously without the ability to distinguish unconscious patterns from authentic expression? Someone who firmly believes they must "be themselves" without practicing self-observation will inevitably fall into the trap of repeating old patterns, mistakenly thinking that they fully express their uniqueness this time.
We often say "Be yourself" when we see someone struggling with an idealized image they have created of themselves. In other words, we encourage them to let go of the unconscious "shoulds," rules, and prohibitions they have adopted and instead choose a more spontaneous and less dependent mode of expression. However, this encouragement is futile if we do not advise them to learn to observe their thoughts (including the "shoulds," rules, and prohibitions) created by their conditioned mind.
For example, imagine encouraging teenagers to "be themselves," to express themselves without caring about others' opinions. Aside from the inherent fear of rejection and the need for belonging, it is nearly impossible for them to do so. At the first opportunity for self-expression, they will likely unconsciously repeat behaviors or reactions they have adopted from others (such as parents, friends, or celebrities).
Being yourself does not mean expressing whatever comes to mind. Beliefs shaped by upbringing, education, and culture will influence what you perceive as an expression of your authentic self. Forming your identity based on thoughts and behaviors, enhanced with a sense of uniqueness, pride, rebelliousness, and entitlement, does not define who you truly are.
You are yourself not when you merely repeat your conditioning but when you live consciously, aware of the unconscious patterns that inevitably operate within you. This allows your true character to express itself freely, without fears and beliefs of inadequacy pushing you to serve an idealized image. Authentic self-expression emerges when you consciously embody the essence of life itself through the temporary form you have taken.