I go through phases of a week or so where this will happen and the way I get out of it is like the above poster said, force myself to find a distraction. Doesn't matter what, so long as it breaks the habit of worrying because it just gets worse, and you start to worry about worrying.
Films, exercise, tv, a book, a game or puzzle, eat something, sort something, anything really. Personally I like to try and learn a new guitar tune or play bejeweled or clean my flat, your experiences may vary.