A 34-year-old male software engineer presents to the psychiatric clinic with a 5-day history of persistent headache, fatigue, irritability, difficulty concentrating, and depressed mood. He also reports feeling anxious and jittery. He states that 6 days ago he abruptly stopped drinking coffee after consuming approximately 600 mg of caffeine daily for the past 4 years because his cardiologist recommended caffeine cessation due to premature ventricular contractions. He reports that his symptoms began approximately 12 to 24 hours after his last cup of coffee and have been worsening. He denies any prior psychiatric history, substance use other than caffeine, and these symptoms were not present before he stopped drinking coffee. He has no history of anxiety or depressive symptoms. His vital signs are within normal limits, and his medical workup is unremarkable. What is the MOST likely diagnosis?