A 28-year-old woman with newly diagnosed sputum-positive pulmonary TB is registered under RNTCP. She is on standard first-line therapy (HRZE for 2 months followed by HR for 4 months). She reports irregular attendance at the TB clinic and admits to missing doses occasionally. At the 3-month follow-up visit, her sputum smear microscopy shows 2+ AFB positivity. What is the most critical intervention to prevent treatment failure and drug resistance in this patient?