"Questioning yourself is hard, convincing yourself is harder.", said the guy while pounding his head to a wall...
PS. I am not philosophical now... I have a qualifying exam in two weeks and the usual preparation is just to reiterate what you know with yourself and question every tiny detail to find out what you don't know good enough and try to fix/hide that part in the presentation. The process is sometimes mind blowing since you cannot bullshit yourself easily!