How did Briony's need for control lead to her not fully being able to atone for her wrongdoings?

Briony is our narrator and a writer in Ian McEwan's Atonement. As a child, she is described as someone who is "possessed by a desire to have the world just so" (4). She is characterized by her "love of order" and a "passion for tidiness" (7). Therefore, it makes sense that she is a writer who can create and order her own imagined worlds in a way that she cannot control the real world in which she lives.
At the end of the novel, in the section that takes place in 1999, Briony is an old woman and a successful writer. She reflects on her best-known work, which is basically the story we just read about her, her sister Cecilia, and Cecilia's lover Robbie. Since Briony is the writer of the story we've just read, she has controlled the story so that it works out how she wants it to. It is not until this epilogue that we learn that the events Briony described near the end of the novel did not actually happen. Cecilia and Robbie did not end up together; they both died during the war. The "happy ending" Briony wrote was a strategy she used to try to atone for her sins, primarily the sin of accusing Robbie of rape and separating Robbie and Cecilia forever.
In the novel's epilogue, Briony writes, "The problem of these fifty-nine years has been this: how can a novelist achieve atonement when, with her absolute power of deciding outcomes, she is also God?" (350). Most sinners confess their sins to God, through a religious clergyman; if the sinner is contrite, God grants the sinner atonement. Briony, in controlling the narrative, is also the "God" of the narrative, its creator. Her question means, how can she ask for and receive forgiveness when she can only ask it of herself? She needs the forgiveness of her sister and Robbie, but since they are dead, she can never achieve that. We realize here that even though she has attempted to make up for her wrongdoings, she never really can gain the atonement she desires.