There is no surefire way to make your brain stop thinking about something, but there are things that you can do to make the thoughts less severe.
First, take a closer look at the thought that is bugging you. Is it really that bad? Sometimes our minds just magnify our worries. By focusing on our worries we can think about their solutions.
Second, talk to people about the things that bother you. They may never understand you completely, but letting out your feelings can do you a lot of good. Who knows, maybe you can pry some good advice from the people you talk to.
And finally, do something about it. The more you ignore the things that worry you, the bigger they become. Confront rather than evade.