Sometimes, it really does boil down to a "chemical" thing. The right medication can be a real game changer. On the other hand, it can also be a matter of making YOUR "reality" what you want it to be - making a conscious choice to focus on what you want to focus on, and ignoring / banishing what you don't. As the others said - finding a distraction when things are pulling you down, to refocus your mind when it starts that seemingly endless circling, can help.