Worrying persistently is a common behavior that can be expressed in a variety of ways. One way to express this behavior is to OBSESS, which is defined as to think about something constantly and often anxiously. Another way to express this behavior is to BROOD, which is defined as to think deeply and at length, especially in a mood of melancholy or depression. Lastly, another way to express this behavior is to NAG, which is defined as to persistently criticize or complain.