When I used to stress about exams in college, I used to try to calm myself down by thinking about it analytically, about how, say, five points on a test could influence my entire life.
Making a lowball extimate, there's a 0.1% chance that five points on any given test will influence my final grade.
And there's maybe a 0.01% chance that any one particular grade will influence what graduate school I get into.
So that's a 0.00001% chance of five points on any one test influencing my life in a major way.
Unfortunately, this didn't calm me down much, because while that number seems pretty low, it's actually very high, considering how many exams I used to take over the course of college.
I guess the moral is that one little thing can change your life in a big way, but it probably won't.