Guy de Maupassant's short story "The Necklace" examines the desires of its protagonist, Mathilde Loisel, an attractive and charming woman who feels burdened by her lack of wealth and low social status. Mathilde wants only to attend one of the lavish parties that represent the life of luxury she is excluded from.
However, after her husband gets an invitation to the Minister of Public Instruction's party at the palace of the Ministry, Mathilde becomes irritated, claiming that she could not possibly attend because she has nothing suitable to wear. Her husband helps her buy an expensive dress, only for Mathilde to further complain that she has no jewelry. Mathilde manages to borrow a fancy diamond necklace from her friend, Madame Forestier, but loses the valuable at the party.
Rather than confessing her error, Mathilde and her husband go into deep debt to replace it. Only after many years is it revealed--upon Mathilde finally telling the truth to Madame Forestier--that the necklace was a fake made out of paste. In this way, there is no true exterior antagonist within the story; the greatest opposing force is Mathilde's selfishness, greed, and lack of gratitude. Mathilde is, in fact, her own enemy.
First of all, do you know what the terms in the question mean? The protagonist is the character the story is centered around, or the main character. In longer stories or novels, there may even be more than one. I like to look at the protagonist as the one who changes or is changed by the events in the story. The antagonist is the person or force that stands in the way of what the protagonist wants or tries to do. Sometimes, the antagonist is the same person as the protagonist, or a part of that person.
In "The Necklace," no-one is working against Madeleine, per se, but her own dissatisfaction with her place in life (or her envy) gets in the way of her happiness. You could say that she is both.