I’m pretty sure the title is one word, “The ______ “. I remember most the ending; a little girl is walking around the perimeter of her house (a Victorian?), looking in the windows, but can’t get in and no one can hear/see her. Through the windows she sees another girl who looks exactly like her, interacting with her (the main character’s) family, playing with her toys, basically living her life in her place like an impostor. At the end she discovers she is in fact dead, a ghost, and the other little girl has replaced her.