Pegasus many of us fall into this habit of worrying constantly,but just as the brain has got set into this pattern of thinking,it can also be trained differently;like Open Eyes above has pointed out.It would also help to get your Thyroid checked.
So far as breaking this cycle of worrying is concerned,please go through the link below,it may help you
http://jeeteraho.blogspot.in/2012/02...happiness.html