know one's mind → ξέρω τι μου γίνεται, ξέρω τι θέλω
know one's own mind → ξέρω τι μου γίνεται, ξέρω τι θέλω
know one's own head
To clearly understand one's own feelings, intentions, preferences, etc; to know precisely what one wants; to be decisive or determined.
https://en.wiktionary.org/wiki/know_one's_own_mind