A 40-year-old female patient has been observing excessive menstruation accompanied by spasmodic pain in the lower abdomen for a year. Bimanual examination performed during menstruation revealed a dense formation up to 5 cm in diameter in the cervical canal. Uterus is enlarged up to 5-6 weeks of pregnancy, movable, painful, of normal consistency. Appendages are not palpable. Bloody discharges are profuse. What is the most probable diagnosis?