After Mathilde loses the necklace, she learns what hard work really is. Prior to this, she had wasted her time, pining for a more luxurious life. Perhaps if she would have worked as hard as she does after losing the necklace, she might have been able to occasionally spoil herself with some small luxuries. Upon losing the necklace, she must work with her husband to repay the debt. There is no mention of her spending her time daydreaming about luxuries she doesn't have. She just works and perhaps truly learns what a struggle it is to work hard and still remain poor.
If she would have found the necklace, she would have returned it to Madame Forestier. There is the possibility that the necklace would be returned and nothing more would be said. However, suppose that Madame Forestier tells Mathilde that the necklace is a fake. Mathilde might be offended. But, maybe she would then consider that it couldn't have been the necklace that changed her attitude and made people flock to her at the ball. Maybe she would realize that the change in her attitude was why she had such a good time. Recall that she is wearing a nice dress but one within their price range. And she is wearing a necklace made of paste. It is her attitude that had changed. This could or could not have changed her outlook on life.
The other possibility is that she returns the necklace, still believing it is made of diamonds. In this case, it seems likely that she would go back to her old ways of dreaming of a more luxurious life, of dreaming of the next diamond necklace.