While I haven't actually read it, I believe that the short story "The Likeness of Julie" by Richard Matheson might be what you're looking for. According to the review at http://www.helium.com/items/1900774-the-likeness-of-julie-richard-matheson it involves a plain, unappealing girl who the main character can't stop himself from becoming obsessed with. Apparently one of the stories in the film "Trilogy of Terror" is based on it.