A 29-year-old woman presents with recurrent episodes of palpitations, described as sudden onset, rapid, and regular, lasting 10-20 minutes and resolving spontaneously. She feels lightheaded during episodes but has never lost consciousness. An ECG recorded during an episode shows a regular narrow complex tachycardia at 180 bpm with no clearly visible P waves. Carotid sinus massage terminates the rhythm. What is the most likely diagnosis?