A 42-year-old female reports 3 years of mood fluctuations. She describes periods lasting 1-2 weeks of feeling "up" — increased energy, decreased need for sleep, and increased talkativeness — alternating with periods of low mood, fatigue, and hopelessness lasting 2-3 weeks. During the "up" periods, she does not meet full hypomanic criteria (fewer than 3 additional symptoms, no unequivocal change in functioning observed by others). She has never had a period meeting full criteria for a major depressive episode or a hypomanic episode. She has not been symptom-free for more than 2 months at a time. What is the MOST likely diagnosis?