People don't get the movie, and are confused, but I can explain
OK people are not getting this movie at all. So let me explain it real quick.
Its simple. Joanne (Sarah Michelle Gellar) and her father get into a car accident when she was only 11 years old. Joanne dies on the scene. The car that crashed into Joanne's father's vehicle, had a man and his girlfriend, Annie. The girlfriend had been brutally beaten before the accident by a man that wanted to rape her. She dies soon after the accident while only been a few feet away from Joanne. Annie's soul leaves her body and somehow enters Joanne's dead body. Joanne's eyes open and is in shock, probably because Annie is extremely confused.
I'm sure that once Annie entered Joanne's body she quickly took over Joanne's memories and completely forgot about her own. Still, Annie's memories began to make their way out and tried to mix with Joanne's memories. That's why she began to see everything that had happen on that final night of her life as Annie.
Annie got a second chance in life as Joanne, and she grew up thinking she was Joanne. At the end of the movie, after she kills her attacker, Annie realizes who she is and what had happen. She realized she was Annie in Joanne's body and she had been given a second chance.