Memoirs of a teenage amnesiac - Gabrielle Zevin

When Naomi tries to piece back the fragments of the last three and a half years of her life, she discovers a lot. She has a boyfriend but can't remember him, her mother and father are divorced, and she has forgotten that she is supposed to hate her mother. She also has a group of friends which simply doesn't seem that attractive any more and, despite having meticulously kept a diary, she only mentioned what she ate every day in it!

But it's not all bad, because 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 be the same?