Think about it; if you were to express yourself in all the identities you maintain, you might feel schizophrenic. What are your separate identities? Do you use a different voice in each role you play in the world? I'll bet you do. You don't use the same voice to speak to your significant other that you use when you talk to your child. Does this mean you are all these different people, or that there are various facets to your 'personality'? (another vexed term for postmodernists).
So if we contain multitudes, as Walt Whitman presciently put it, then how can we be held to one voice? Where do we get the idea that this one individual identity exists? Well, most recently, from the Modernists and the Romantics before them. We have inherited the notion that the individual reigns supreme. We have long forgotten the nameless, faceless, seemingly identity-less days of the Medieval period, or the Dark Ages, when an anonymous monk toiled in silence over his book of psalms. And we don't want to go back to those days, either. We want authorial rights, and we want them now!
If you spoke in all your various voices that you use during the course of the day, would you sound crazy to yourself? Would you doubt that you have one unified identity? Try thinking this way and see what happens. Better still, try writing down what each persona you live with would say, and all the different ways you have to say something, thinking specifically of your audience and the context in which you're likely to say it, and realise that you contain many voices. And that there's nothing wrong with that.
Sorry I complicated this seemingly uncomplicated subject. I have a habit of doing that. ;-)