Agreed. It's lazy and dismissive advice. I wish it wasn't such a trope in children's entertainment.

Most people don't have a fixed sense of self until adulthood. And wise adults understand that there's no self at all.

I reinterpreted it as "be the you you would be if you could control who you are", but that doesn't exactly roll off the tongue

>wise adults understand that there's no self at all

Except, of course, for Carl Jung. And Polonius in Hamlet. But that's all.