"Someday My Prince Will Come" follows the story of an 11 year old rather than a girl who is the normal age to be married. Also, in the film the girl is left without a prince at the end whereas most fairy tales end happily. She is however, still hopeful though she has had a few boyfriends over the course of about a year and none have turned out well.
The movie is also set in the present day which is unlike most fairy tales. All of the children seem to come from low income families. This is unlike a usually fairy tale because generally the prince in the story would be wealthy and would rescue the princess/girl from her humble existence. The children also use profanity freely and the princess herself is not pious and admirable like the princesses in fairy tales. In the movie Laura-Anne swears at and calls the boys who hurt her bad names. Maybe this is the big difference between her ending and the ending of most fairy tales. Since Laura-Anne has not behaved properly like the princesses in the fairy tales, she does not get the happy endings those princesses do.