I think it depends on the intensity of it, in my case. Its simple. Get yourself into a relaxed state of mind, clear out all negative vibes, open yourself to some positive thinking, breathe and relax and repeat a positive statement about yourself or something over and over. Repeat it in time with your breathing. Any simple affirmation works. Do it for about 1 minute then slowly build on that til youre doing it for several minutes. You can do it several times in a day as often as you like. Eventually you will retrain your thought patterns to wander positively instead of negatively. Like I said, it takes time and repetition. Ive heard 3 weeks to alter an unhealthy habit but continuing to do this will bring greater benefit that lasts longer. Its not for everyone. Some people are sadly so down that even this wont work. but it helps some.