Memoirs of a Teenage Amnesiac by Gabrielle Zevin

Did you ever wonder what it would be like to not remember yourself? What would you be like if you didn't remember all your hang ups and bad experiences? Would you suddenly be a nicer person? More forgiving? More social? Interested in different things? Naomi Porter got that chance at the beginning of her senior year of high school. She cracked her head on the school's front stairs and lost four years worth of memories. Her re-introduction to her old world, her old friends, her old crowd isn't smooth so Naomi reivents herself; but then one day her memory comes back...