Book — Gabrielle Zevin

After an accident that leaves her with partial amnesia, Naomi tries to piece together the fragments of the last three-and-a-half years of her life. She discovers that she has a tennis-champion boyfriend but can't remember him, is co-editor of the yearbook with a quirky guy who wears a smoking jacket, her parents are divorced, and she apparently hates her mother. She has friends who simply don't seem that attractive any more and, despite having meticulously kept a diary during the now-lost years, she only wrote about what she ate every day in it!
But when a girl loses three-and-a-half years, she gets a chance to reinvent herself. After all, who is to say that everything has to stay the same?