Let's talk about "being yourself". As in, "you just aren't yourself lately". Or, "just be yourself, and you'll be fine". What does that even mean? Who else would you be behaving as, if you weren't behaving as yourself?
Phrases like that frustrate me to no end. The "self" to which they refer isn't actual internal thoughts and tendencies, but outward perception. When someone tells you that "you just haven't been yourself lately", they don't refer to a change in your mental state, but a change in the patterns of behavior they perceive.
Assuming that there aren't actually aliens that constantly monitor human brainwaves, thoughts are completely private. We aren't even cognizant of most of our own thoughts. They are the true definition of "self". As long as you are thinking, and thus, existing, you can be no one but yourself. You can present a facade that is not your innate self, you can conceal your self, but you cannot actually be any other self but your own.
Because I'm infatuated with paradoxes, and feel a compulsive need to point them out, here you go: Although people are constantly evolving (or devolving) in their mental state, which we refer to as the self, this evolution of self is actually an innate part of the constant self. Change is a part of consistency.