It depends on why you are worrying and in what way. And also maybe on your religion/beliefs.
I'm worried about a exam.
When I was in college, I would study for a hour and take a break. Clear your mind for a minute. Focus on the material that you are struggling with rather than the stuff you know. Then give yourself on last review on everything. Cramming doesn't help.
My philosophy is if you can't change it right now, no need in sweating about it. People, money, life..Sh** happens we can't control. Work with what God has given you, and pray on the rest. Just keep it moving. Focusing on what you can't control will drive you crazy