We Were Liars

The book “We Were Liars” by E. Lockhart is about a rich family who owns a private island off the coast of Manhattan called Beechwood.
One summer the main character Cady, returns from her the island suffering from various illnesses and recovering from selective amnesia. This amnesia is related to an accident that happened that same summer. During her time on her family's private island Beachwood, she goes through clinical depression and has a large list of disorders, because she is the only narrator how can we be certain that she is reliably telling the story?
When Cady describes her injury she said, "A truck is rolling over the bones of my neck and head. Vertebrae break, the brains pop and ooze. A thousand flashlights shine in my eyes. The

Anterograde amnesia is usually caused by a large force applied to the back of the head which causes memory loss of the accident, and the inability to create new memories after the accident. However, people who suffer from Anterograde amnesia can still remember what happened before the accident. But, Cady could forge new memories after the accident, so we can rule out Anterograde amnesia as the condition she is suffering. The second possibility, PTSD is caused by a traumatic event that has happened. The symptoms of PTSD

One thing we can see is acting out the trauma through play, stories, or drawings. After the incident, Cady begins to write stories about three princesses and their father. We have no evidence that she has been writing stories before she has experienced the accident. This is one of the symptoms of PTSD, which is acting out the trauma through play, stories, or drawings. Since PTSD does not usually cause amnesia, Cady could also be suffering from PTSA (Post Traumatic Selective Amnesia), a separate strand of PTSD. At the end of the book, we are told that Cady and her friends burned down her grandfather's cottage, Clairmont. They did this because of the relentless fighting of his three daughters on who will inherit their father's property. Yet during the incident, Cadence had accidentally killed her friends Johnny, Mirren, and Gat.
Cady, trying to rescue her love interest Gat, runs into the burning building. Unfortunately, she fails to find him, or her cousins in time. She then jumps from the burning building, as the fire spreads. Cady lands in the ocean and hitting her head on some rocks causing PTSD as well as PTSA. But since she has no memory of that summer she can't remember anything that happened about the incident or the summer that the incident happened