Determine the Worst-Case Scenario
Knowing the worst possible outcome will help put things in perspective. Are you going make your boss disappointed or lose your job? Are you going to get a B- on a project or flunk out of school?
Once you have determined the worst-case scenario, you need to mentally accept it. Assume that it is going to happen. If you are resigned to the fact that the worst outcome is going to come true, then you can stop worrying about that possibility. After you accept the situation, you can move on to the next step.
Improve the Worst-Case Scenario
Now that you are free from worrying about the outcome, you can devote your full attention toward improving it. Try to find ways to reduce the negative effects of the outcome. You may not be able to turn a losing situation into a winning situation, but you might be able to make it neutral or at the very least, less bad.