A 40-year-old woman experienced emotional stress 3 months ago and has since had poor sleep. She suffers from unquenchable thirst, requiring large amounts of water intake (4-5 thermos flasks daily), preferring cold water. Urine output has significantly increased, averaging once per hour, with nocturia exceeding 5 times nightly and total daily urine output reaching 9L. She has developed poor mental state, irritability, weight loss, and headaches in the past 2 weeks. What is the most likely diagnosis?