1) Worrying about something so far out that it might not happen.
The way to stop worrying about future problems is to get more information to understand the problem, and to figure out the next step.
2) Being nervous about the things outside of your control.
As stated, they are outside of your control so there is nothing you can do; the only sensible solution is to stop worrying about them.
3) Worrying about something that you know you need to do, but you also do not want to do, like laying people off, for example.
This is perhaps the most challenging worry as it involves emotions, and emotions are not easily quantified.
To stop worrying about this type of problem, put the emotions aside and think through the logic of the business case first, quantify the business issues. Once you understand why you have to resolve the issue at hand, you will find a way to deal with the emotions of doing it.