There's nothing wrong with thinking so much or even stressing so much. At least you care enough to stress, a lot of people in this world just don't give a fuck - & not in a cool way at all.
Anyway, like others have said, you need to direct that energy somewhere. However, unlike others have said, I'd actually say that rather than directing your energies towards something with little end product like writing or exercising, why don't you direct your energies into changing whatever it is that bothers you so much? If it's something specific - change it. If it's not specific things, then I guess you best get writing or exercising.