Americans are highly motivated to lose weight—as a growing list of best-selling books and highly trafficked dieting Web sites attest. We're just not approaching it the right way. The pressure we put on ourselves to succeed—and the self-criticism we indulge in when we fall short of the mark—can have dire emotional and dietary repercussions.

Consider that pair of jeans hanging reproachfully in the closet. You realize they don't fit, and you feel unattractive and worthless. This tendency to evaluate yourself too harshly will only make you give up altogether. You want to head to the fridge for solace.

You need to identify the things you're telling yourself that cause you to feel discouraged and to throw in the towel. Don't beat yourself up when you overeat. Accept that you acted in a self-defeating way, then establish better methods to meet your goal. Review what you'd like to do and work toward that goal.

Perhaps you're not (yet) berating yourself for failures, but putting inordinate pressure on yourself to succeed. When you tell yourself, "I must lose 25 pounds by Valentine's Day, or I'll never get a date," you're setting yourself up for emotional turmoil, as well as weight-loss failure. Losing weight in a prescribed amount of time is a worthy goal, but the perfectionistic premise that sneaks into your thinking may well interfere with sensible eating and exercise.

In a perfect universe, the sight of those jeans, or the knowledge that Valentine's Day is around the corner, would elicit rational thoughts like, "I'm going to look great soon, and I'm going to enjoy the challenge of eating sensibly and exercising along the way." But few of us think that.

PT spoke with Nando Pelusi and Mitchell Robin, clinical psychologists in New York City, about what we really tell ourselves, sabotaging our own best efforts to lose weight—or meet any goal.

• "I must be thin."

  This creates desperation, which undermines a healthy long-range approach to sensible eating. Also, perfectionism pervades this thinking (I must not only be thin, but also perfect).

• "I must eat until sated."

  Early humans lived in an environment in which food resources were scarce. While our ancestors had to hunt down squirrels and eat them, we can supersize a Whopper meal and skip the workout.

• "I need immediate results."

  The demand for immediate improvement undermines commitment to a long-term goal. Quick fixes are hard to pass up: "This cupcake will make me feel good right now." We think, why bother eating healthfully, when the reward is far off? Dieting requires present-moment frustration and self-denial with little immediate reward.

• "I need comfort."

  People eat to avoid feelings of loneliness, depression and anxiety. Fatty and sugary food provides immediate comfort and distraction from other issues. Resolving some of these problems may help you overcome poor eating habits.

• "I feel awful."

  "It's terrible being heavy." For some, being overweight is the worst thing imaginable; it can immobilize you and leave you dumbstruck. That's a reaction more suited to tragedy. Weight loss is best achieved without that end-of-the-world outlook.

• "It's intolerable to stick to a diet."

  "It's just too hard to diet." This thinking renders you helpless. People who are easily frustrated want easy solutions. We're seduced by fad diets because they appeal to that immediacy. Yet people who rely on fads suffer high failure rates. When you diet with the short term in mind, you don't learn strategies that require patience and persistence.

• "I am no good."

  "Because I am having trouble in this one area I am worthless." Being overweight can be viewed as a sign of weakness or worthlessness, and most people aren't motivated when they feel that way. Another form of worthlessness: "My worth is dependent on my looks." This idea confuses beauty with thinness, a concept played out endlessly in the media.