To start off, the novel This Darkness Mine was pretty dark. The main character Sasha Stone seems perfect in the beginning, but she quickly devolves throughout the novel. Her problems are a mixture of physical and mental, and sometimes it's hard to tell which. As always, spoilers ahead.
As a reader, I'm inclined to believe Sasha when she claims that her twin sister lives inside her, even if it logically is not true. When she finds out that she absorbed her twin in the womb, Sasha comes to believe that her heart is actually her twin sister, Shannon. She begins to communicate with Shannon through writing and text, and Shannon sometimes takes over her body. One of these examples is when Shannon takes over and throws Sasha out of the window.
While Sasha is in the hospital, she finds out that she has a heart disorder, and it has not been her supposed twin sister controlling her, but cardiomyopathy. She is moved into a hospital for girls waiting for heart transplants. Here, Sasha meets with her therapist but makes no progress, although she pretends to. She also runs into various problems with the other residents.
Throughout the novel and Sasha's story, she struggles with her parents and friends. The reader has to feel bad for it because it seems like pretty much everyone hates her. While she's in the hospital, her boyfriend cheats on her with her best friend, and he visits but only to break up with her.
Also, her parents blame her for the death of her twin and don't really seem to like her, especially her dad who tells her this to her face. Sasha isn't perfect (she does a lot of awful things), but she's also a girl suffering from mental and physical disorders so it seems like she's treated a little harshly.
It isn't until the epilogue that I become truly convinced that Sasha's sister isn't real. The author does an amazing job of putting the reader in Sasha's shoes and showing how she is truly convinced that her sister lives inside her. After Sasha's heart transplant (where "Shannon" was removed and replaced, although Sasha cuts part of "Shannon" out herself), Sasha learns everything she can about the girl whose heart she has. From this, Sasha convinces herself that this girl now lives inside her and is alive just like Shannon was.
This Darkness Mine comes full circle, but to me it is really sad that no one was able to help her with her obvious multiple personality disorder. To find out more or purchase this novel visit Goodreads.