It was a touching movie! (Spoiler alert)
At first look I didn't understand the ending as well, but to be honest what Diane did, nobody can simply does that! She sacrificed herself, because of her love for her friend, and she found herself unworthy, of the life that she could ever had..she didn't see any poit in being alive and have a life..she had already destroyed her soul, I don't mean it by the abortion, but for the mistakes she had made in her life. I envy her, she got to die as by the time she was ready, but almost nobody get the chane to leave life that easy. So, Diane, as all of her adulthood scenes were only a vision of her future, while she was standing with Maureen and the shooter, found out that there's no point in (her) life so she just asked the shooter to kill her..and she just died..I wish Dieing was that simple..although it was one of the best movies I've eva seen! It is worth watching it!