get (something) through (someone's) head
To make someone or oneself acknowledge, accept, or understand something. Look, just get it through your head: we're moving, and there's nothing that's going to change that now. I don't know how else to get it through her head that I want to break up.
get through one's head
Understand, believe, or be convinced. For example, Bill cannot get it through his head that John is moving out.