My own take on this movie....
I've watched this movie 4 times in as many weeks. (Don't you love satellite tv?)
Anyway - this is my theory....
Danika is reliving some events of her life. What I got out of it was that everyone she trusted betrayed her. Her therapist, nanny, husband and even her daughter with the lying. I wasn't sure about the role of the teacher, but I think she can be seen as a betrayal as well. Danika goes to see her and it seems that the teacher is almost blaming Danika for her daughter's problems. When Danika confronts her about the book her daughter is reading, the teacher says she would never assign such a book, but then tries to lay the blame on Danika instead of reprimanding the child and acknowledging the daughter's role in the deceit.
Then at home, Danika is apologizing to the daughter who still hasn't been punished or taken responsiblity or even accused of lying and the whole teacher conference thing. And her husband is more concerned with the daughter's well being. So Danika is being betrayed by everyone - or at least she perceives it as betrayal. (in real life - in my life anyway - I wouldn't get off that easy if I did what her daughter did.....I would have, at the very least, been grounded for 6 months)
However, she is delusional and thats why she is seeing severed heads & her therapist as the nanny, etc. In her delusions, she can't distinguish or maybe remember the roles these people played in her life.
As for the scene in the motel, it shows her pick up a shard of the mirror and hit her husband with it, but they don't show what happened. And the "nanny" is having way too much of a reaction after Danika grabs her and tells her to stay away from her kids. So it made me think that the guy was seriously hurt. Its also helpful to note that he had absolutely no remorse for the affair or getting caught screwing the chick in front of his wife.
On the way home, she gets into the accident - the kids get killed. Like the detective (or EMT guy) says "how do you live after something like this"....well, she totally lost it and ended up a bag lady. She couldn't handle a "normal" life - and you wouldn't blame her for that. Definitely schizophrenic and suffering from major guilt. On her cart are the rosary beads from the car. (((One thing I couldn't tell though and I'm dying to find out....when they show the contents of the cart there are several newspaper articles or papers. I know one is about the missing girl, but are any of the others about the accident???)))
The whole thing with the family together & walking away from the accident is her wishful thinking - that all is well and they're together for eternity. But she doesn't die and is left to live with it all.