At the beginning of the story "The New Dress," the main character, Mabel, is portrayed as a very anxious and insecure girl. When she takes a look at herself in a mirror, she is appalled by what she sees, and she is overwhelmed with "the sense she had had, ever since she was a child, of being inferior to other people." This feeling of inferiority seems to be particularly pronounced, because when Mabel sees herself in the mirror, she thinks to herself, "No! It [is] not RIGHT."
From the next part of the story, the author suggests that Mabel's sense of inferiority is based mostly on her appearance—or, rather, her own perception of her appearance. She imagines other people looking at her and exclaiming "What's Mabel wearing? What a fright she looks!" From this we can infer that Mabel's feeling of inferiority perhaps stems from the fact that she thinks too much and cares too much about what other people think of her. She is severely lacking in self-confidence.
Mabel herself seems to be acutely aware of this lack of self-confidence when she chastises herself for "her own appalling inadequacy; her cowardice; her mean, water-sprinkled blood." The impression created by this quote is that, rather paradoxically, Mabel despises herself for not having enough courage to like herself.
Mabel also worries that she might, in worrying about what other people think of her, be guilty of being too proud. Indeed she thinks of herself, critically, as "puffed up with vanity," and she accuses herself of giving in to "an orgy of self-love." Thus, Mabel's feeling of inferiority is self-perpetuating. She feels that she is ugly, yet she also chastises herself for thinking too much about how she looks. This awareness of her own pride makes her despise herself even more, which in turn makes her believe more strongly that she must appear ugly to others.