looking for title of a suspense movie about woman that kills family

I watched this movie some time around 2008. The main character is a woman that works at a bank. I dont remember a lot of specifics but she finds that her husband has been unfaithful and kills him and then gets in an accident or something like that and her two kids die. At the end of the movie, she is the crazy woman pushing a shopping cart that you could see at the beginning of the movie. It seemed as she had forgotten or that woman was a vision.