Under This Red Rock by Mindy McGinnis

From award-winning author Mindy McGinnis comes a mesmerizing YA psychological mystery following a teen girl who is grappling with the death of her brother as she starts a new job in the caverns of Ohio-only to become the number one suspect in her coworker's murder. Perfect for fans of Courtney Summers and Kathleen Glasgow.
Neely's monsters don't always follow her rules, so when the little girl under her bed, the man in her closet, and the disembodied voice that shadows her every move become louder, she knows she's in trouble. With a history of mental illness in her family and the suicide of her older brother heavy on her mind, Neely takes a job as a tour guide in the one place her monsters can't follow-the caverns. There . . . she meets Mila. Mila is everything Neely isn't-beautiful, strong, and confident. As the two become closer, Neely's innocent crush grows into something more. When a midnight staff party exposes Neely to drugs, she follows Mila's lead . . . only to have her hallucinations escalate.
When Mila is found brutally murdered in the caverns, Neely has to admit that her memories of that night are vague at best. With her monsters now out in the open and her grip on reality slipping, Neely must figure out who killed Mila . . . and face the possibility that it might have been her.