A 38-year-old woman from Mumbai presents with a 6-month history of progressively heavy and prolonged menstrual bleeding (duration 8–10 days, soaking 10+ pads daily). Her cycles are irregular, occurring every 21–35 days. She has severe dysmenorrhea requiring high-dose NSAIDs and occasional work absence. On examination, the uterus is tender, boggy, and uniformly enlarged to 14 weeks size. Pelvic ultrasound shows a heterogeneous, thickened myometrium with indistinct endometrial–myometrial interface and no discrete masses. Hemoglobin is 8.5 g/dL. What is the most likely diagnosis?