We’ll Never Be Apart by Emiko Jean

Murder. Fire. Revenge. That’s all seventeen-year-old Alice Monroe thinks about. Committed to a mental ward at Savage Isle, Alice is haunted by memories of the fire that killed her boyfriend, Jason. A blaze her twin sister Cellie set. But when Chase, a mysterious, charismatic patient, agrees to help her seek vengeance, Alice begins to rethink everything. Writing out the story of her troubled past in a journal, she must confront hidden truths. Is the one person she trusts only telling her half the story?