The question is not grammatically correct in either case. It is either "How can I really know myself?" or "How can I really know *you*?". Adding "self" indicates a reflexive (mirrored) action, so you can't reflect something to someone else (like "know"), one can only do it to one's self.

Maybe you are right, must first clear, then can better understand

First, we have to clarify which do you want to know. Is it yourself or someone else?

Wait you mean how can i really know myself, or how can i really know someone else or how can i know you. im unsure if how can i really know youreself works in the english language but it does sound dumb. yourself is commonly used in situation like calm youreself or you cant keep yourself down.