In the Mouth of Madness

Dr. Wrenn visits John Trent, a patient in a psychiatric hospital, and Trent recounts his story:
Trent, an insurance investigator, has lunch with a colleague who preps him on his next assignment: investigating a claim by New York-based Arcane Publishing. During their conversation, Trent is attacked by a man wielding an axe who, after asking him if he reads Sutter Cane, is shot dead by a police officer before he can harm Trent. The man was Cane's agent, who went insane and killed his family after reading one of Cane's books.
Trent meets with Arcane Publishing director Jackson Harglow, who tasks him with investigating the disappearance of popular horror novelist Sutter Cane, and recovering the manuscript for Cane's final novel. He assigns Cane's editor, Linda Styles, to accompany him.
Linda explains that Cane's stories have been known to cause disorientation, memory loss and paranoia in "less stable readers". Trent is skeptical, convinced that the disappearance is a publicity stunt. Trent notices red lines on Cane's book's covers which, when aligned properly, form the outline of New Hampshire and mark a location alluded to be Hobb's End, the fictional setting for many of Cane's works.
They set out to find the town. Linda experiences bizarre phenomena during the late-night drive, and they inexplicably arrive at Hobb's End in daylight. Trent and Linda search the small town, encountering people and landmarks described as fictional in Cane's novels. Trent believes it all to be staged, but Linda disagrees. She admits to Trent that Arcane Publishing's claim was a stunt to promote Cane's book, but the time distortion and exact replica of Hobb's End were not part of the plan.
Linda enters a church to confront Cane, who exposes her to his final novel, In The Mouth of Madness, which drives her insane; she begins embracing and kissing Cane passionately. A man approaches Trent in a bar and warns him to leave, then commits suicide. Outside the bar, a mob of monstrous-looking townspeople descend upon him. Trent drives away from Hobb's End, but is repeatedly teleported back to the center of town. After crashing his car, Trent awakens inside the church with Linda, where Cane explains that the public's belief in his stories freed an ancient race of monstrous beings which will reclaim the Earth. Cane reveals that Trent is merely one of his characters, who must follow Cane's plot and return the manuscript of In The Mouth of Madness to Arcane Publishing, furthering the end of humanity.
After giving Trent the manuscript, Cane tears his face open, creating a portal to the dimension of Cane's monstrous masters. Trent and Linda flee down a long tunnel as monsters chase them. Linda offers to stay behind to delay the unseen monsters, and because she has already read the entire book. Trent continues running and suddenly finds himself lying on a country road, apparently back in reality. During his return to New York, Trent destroys the manuscript. Back at Arcane Publishing, Trent relates his experience to Harglow. Harglow claims ignorance of Linda; Trent was sent alone to find Cane, and the manuscript was delivered months earlier. In The Mouth of Madness has been on sale for weeks, with a film adaptation in post production. Trent is arrested after he murders a reader of the newly released novel, who has altered eyes and a nosebleed; Trent asks if he is enjoying the book, and when the dazed reader nods, Trent tells him he should not be surprised before swinging the axe.
After Trent finishes telling his story, Dr. Wrenn judges it a meaningless hallucination. Trent wakes the following day to find the asylum abandoned. He departs as a radio announces that the world has been overrun with monstrous creatures, and that outbreaks of suicide and mass murder are commonplace. Trent goes to see the In the Mouth of Madness film and discovers that he is the main character. As he watches his previous actions play out on screen, including a scene where he insisted to Linda "This is reality!", Trent begins laughing hysterically before breaking down crying; finally realizing he was a character in the book all along.

Question: What does Linda admit to Trent?