The Book:
Before I Go To Sleep by S.J. Watson, 2011
The Summary (from Goodreads):
Christine wakes up every morning in an unfamiliar bed with an unfamiliar man. She looks in the mirror and sees an unfamiliar, middle-aged face. And every morning, the man she has woken up with must explain that he is Ben, he is her husband, she is forty-seven years old, and a terrible accident two decades earlier decimated her ability to form new memories.
Every day, Christine must begin again the reconstruction of her past. And the closer she gets to the truth, the more unbelievable it seems.
The Spoilers:
Christine has amnesia after an accident and can’t remember anything every morning. She starts to mistrust her husband after finding a journal she’d been keeping. She reconnects with an old friend, Clair, and learns she had been cheating on her husband. Christine and Ben go on a vacation, and she learns that Ben is not her husband. He was the man that she had cheated on her husband with, who had attacked her and caused her memory loss. He tried to kill her because she was pregnant with his baby but was pretending it was Ben’s. Clair and the doctor Christine was seeing figured out where she was staying and saved her before fake Ben could kill her; she was reunited with the real Ben and her son.