Jackie is a 57 year old accountant who has a 3 month history of fatigue and has presented as she is struggling to concentrate at work. She has an appraisal upcoming and admits to increased stress in the workplace as well as a difficult break up with her boyfriend. She has little appetite and sometimes skips meals. Despite the difficult recent months, she enjoys relaxing with friends and socialising with work colleagues during the day. She denies any abdominal pain, rectal bleeding, or bloating, but admits over the last 6 months she is often constipated. There is no polyuria, polydipsia, or muscle weakness. Past medical history is unremarkable.
Based on the history alone, what is her most likely diagnosis?