You could write about the way the status of Madame Loisel only appears to change through the wearing of the necklace and about how her confidence is false or only fleeting, a bit like Cinderella's - it is all due to change very shortly.
She has four things in her favor at the Ball - two attributes are real and hers alone (her beauty,her lively bright personality) but two are false (a ball gown they couldn't afford and a necklace which doesn't belong to her, and which itself later turns out to be fake.)
Think about these things:
What is Madame Loisel's first physical reaction to the necklace?
What is she afraid of if she goes to the Ball 'unprepared?'
Explain in the detail the sentence where she is pretty and popular at the Ball. How is she feeling inside? How does the necklace contribute to that? Does it make her 'feel like a million dollars?' Is it because she has temporarily elevated her status to that of the other girls?
Finish by commenting on her level of confidence as she scuttles like a drab poor church mouse through the Champs Elysee.
At the beginning of the story, Madame Loisel is a haughty, proud woman whose pride leads her to be discontened and grumpy about her life. She is absolutely positive that she is destined for more. Her supreme confidence in her slighted position in life leads her to complain, be unhappy, and not appreciate what she does have. By the end of the story, however, her confidence and pride has been humbled. She approaches her old friend with total embarrassment and humility about her position in life. She is well aware that she appears to have aged significantly, of her roughed, work-hardened skin and lined face, of her ragged clothes. She isn't as confident as she used to be. Life has humbled her; working off the debt for the necklace has lowered her confidence and pride to fit her actual station in life.
Keep those things in mind as you write your analysis. For the topic sentence, try to condense the thoughts above (if you like and agree with them) into just one concise sentence. When you write the analysis itself, it helps to read through the story itself and find quotes that support Loisel's change of confidence and pride throughout the story. Try to work in a sentence or two from the story that shows how prideful and unhappy she was at the beginning of the story, then another sentence or two about her lack of confidence at the end. That will work as a good contrast that will help to support your assertions. I hope that those thoughts helped to get you started; good luck!