Worry is the emotional distress that one causes oneself by thinking about something distressful that might happen at some future moment. The thoughts you think in the present create your present feelings. When the thoughts on your mind are pleasant or joyful thoughts, your entire inner system reacts accordingly. Your breathing rate changes, your brain waves change, your blood pressure change, your energy level changes, your hormones change, you heart beat changes, your immune system changes, your muscle tension changes, your facial expression and posture change, and your tone of voice changes. All these change counterproductively when you needlessly cause yourself anxiety by thinking worrisome thoughts. These changes happen instantly.
Now in your inner mind, tell yourself, "Every time I begin to worry, I will immediately think of something to be grateful for." Repeat this enough times until you feel that your inner mind will automatically go into gratitude mode as soon as it's aware that it's in worry mode.
(From Rabbi Zelig Pliskin's book: Thank You! Gratitude: Formulas, Stories, and Insights: Artscroll Publishers)