Can someone please explain the ending to me?

Don't read any further unless you are prepared for spoilers. The only scene that takes place in the present is the scene in the rest room. The other stuff is either flashbacks of the growing friendship between Diana and Maureen, or Diana's imagined future flashing before her eyes as she wrestles with her conscience in the rest room. And if you watch the flashbacks you can see the process of Diana becoming less and less selfish; a maturing that makes it difficult for her to allow Maureen to sacrifice her life so that Diana can live.

Emma does not represent the aborted child but rather the child that Diana imagined that she would conceive "responsibly" had Maureen been the one who was killed. She and Maureen had picked out girls' names together very early in the film, when they were imagining their future lives.

When watching movies like these, you HAVE to pay attention. The 2nd poster explained it in a very simple way.

Okay, so we see Diana's life as an adult. What we don't realize until the end of film, is that all of this was just an imagination. The movie continuously goes back to the bathroom where it leads to Diana making her final decision, for a reason. She was sort of looking at her life in the future. (which makes the title of the movie straight to the point). This is how her life would have been if she wouldn't have sacrificed her life.

Now if you notice, her life as an adult is pretty miserable. She has an enormous amount of guilt. She also doesn't trust her husband, who she thinks is cheating on her, because she sees him walking down the street with a young girl. Now I must admit, that part did confuse me a bit. I couldn't tell if that was supposed to be an older Emma walking with him, or Diana herself... since she met him when she was a teenager. (He made a speech at her high school one day).

The young Diana can forsee this miserable life. This feeling of guilt that she will have for the rest of her life, if she does not do what is right.

Now, this "new" Diana only makes such a tough decision to give up her life, because several events have matured her greatly, and we can definitely see the difference in her attitude leading up to the day. The abortion for one. Now let me make something clear... the aborted child is not Emma. This is also, another imagination. This is just young Diana imagining having the little girl that she always wanted. (we know this because she and Maureen are picking out their names for their future children). I couldn't tell if the young Diana was turing down the guy in that one scene, but I think she did, which is something that the old, out of control Diana would not have done. And lastly, the biggest reason that Diana makes her tough decision- is Maureen, herself. In a bunch of flashbacks, you see how close Maureen and Diana had become. Diana even questions why she was given such great friend like Maureen. Diana knew that she had to let Maureen live her life, because she knew that Maureen truly deserved it.

In the end, Emma runs away froms school. Diana is searching in the woods for her, but then Emma disappears. This only means one thing... time has caught up. The film is now in the present time again with Diana, Maureen and Michael Patrick in the bathroom. Diana makes her final decision to give up her life.

Maureen now has the perfect life that was miserable for Diana, because she deserved a life like this, and Diana gave it to her. Why? Because Maureen gave her what Diana appreciated the most, a true, loving friendship.

Then the film briefly goes back to the adult Diana taking flowers to the school- the woman asks her if she's a surivor, and she says no.

Umm..yeah! This is just my take on it, from watching the film a few times.

Live for today, not tomorrow, because tomorrow might be too late.

What a preposterous imagination this girl had! The dumb twist ruined this film.

SPOILERS

I must say I didn't have it all figured out until reading what everyone else had to say. Just to add something to the idea that Diana is the one that died in the end: Don't forget about that scene (I think it was near the end) when Diana's mother rushes up to the school and screams "Diana!" I think they probably have told her that Diana was shot/killed.