A 22-year-old woman presents with dizziness and fatigue for 6 months. She reports menorrhagia lasting 7-8 days with blood clots for the past 2 years. Laboratory tests show RBC 3.0×10¹²/L, hemoglobin 65g/L, serum ferritin 10μg/L, serum folate 16 ng/ml, vitamin B₁₂ 600 pg/ml, and reticulocytes 0.015 (1.5%). The most likely diagnosis is: