I am hardly in a position to respond to this with the well-thought-out response it deserves, but I do think a lot of the cute, trite phrases such as "be yourself!" and "try your best!" make very, very little sense when actually thought about. Because you're right -- you can't not be yourself. Every action you take is unified by the "I" of your conscious and that's really the only unity, the only "yourself" there is. Saying "you should stop trying to force yourself to be [X person]" makes a lot more sense.

As for "try your best", well, I try my best quite often and most of the time it ends up not being good enough. By "trying my best" people seem to be saying I should be as good as they think my best should be, not my actual best.

I am not really a cynical person, but I don't have much patience for these phrases. At any rate, good luck with your exams.