Bridget Jones is not alone in turning to self-help mantras to boost her spirits, but a study warns they may have the opposite effect.
Canadian researchers found those with low self-esteem actually felt worse after repeating positive statements about themselves.
They said phrases such as "I am a lovable person" only helped people with high self-esteem.
The study appears in the journal Psychological Science.
A UK psychologist said people based their feelings about themselves on real evidence from their lives.
The suggestion people should "help themselves" to feel better was first mooted by Victorian Samuel Smiles 150 years ago.
His book, called simply "Self Help", sold a quarter of a million copies and included guidance such as: "Heaven helps those who help themselves".
Self-help is now a multi-billion pound global industry.
The researchers, from the University of Waterloo and the University of New Brunswick, asked people with high and low self-esteem to say "I am a lovable person."
They then measured the participants' moods and their feelings about themselves.
In the low self-esteem group, those who repeated the mantra felt worse afterwards compared with others who did not.
However people with high self-esteem felt better after repeating the positive self-statement - but only slightly.
The psychologists then asked the study participants to list negative and positive thoughts about themselves.
They found that, paradoxically, those with low self-esteem were in a better mood when they were allowed to have negative thoughts than when they were asked to focus exclusively on affirmative thoughts.
Writing in the journal, the researchers suggest that, like overly positive praise, unreasonably positive self-statements, such as "I accept myself completely," can provoke contradictory thoughts in individuals with low self-esteem.
Such negative thoughts can overwhelm the positive thoughts.
If people are instructed to focus exclusively on positive thoughts, negative thoughts might be especially discouraging.
The researchers, led by psychologist Joanne Wood, said: "Repeating positive self-statements may benefit certain people, such as individuals with high self-esteem, but backfire for the very people who need them the most."
However, they say positive thinking can help when it is part of a broader programme of therapy.
Simon Delsthorpe, a psychologist with Bradford District Care Trust and spokesman for the British Psychological Society, said self-esteem was based on a range of real life factors, and that counselling to build confidence - rather than telling yourself things are better than they are - was the solution.
"These are things like, do you have close family relationships, a wide network of friends, employment and appearance.
"If you're not close to your parents, don't have many friends, are unemployed and are unhappy with your appearance, it might be hard to have high self-esteem.
"But if your experience is the reverse of that it would be much easier to say 'I'm OK' and believe that."
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