The Forgotten (2004)

A grieving mother, Telly Paretta, struggling to cope with the loss of her eight-year-old son, is stunned when her psychiatrist tells her that she has created eight years of memories of a son she never had. But when she meets a man who has a similar experience, Telly embarks on a mission to prove her son's existence and her sanity.

Official website: http://www.sonypictures.com/movies/theforgotten