exams make me very stressed

i never have enough time to fully comprehend all the material. hell, most of the time I can't even go over all of it even once. Then I go take the exam feeling like an idiot but so far I've always passed.

it feels like to fully understand an area of math I have to study math 2 levels deeper/harder/more abstract. i.e. I'm only becoming comfortable with ODEs (though I've passed that course some time ago) after taking a PDE course.

is this just how math is, or am I supposed to put more time into studying earlier in the semester