A 35-year-old man with bipolar I disorder presents with a new-onset manic episode and is successfully treated with medication adjustment. He notes chronic depressive symptoms that, on reflection, long preceded his manic episodes. He describes these symptoms as "feeling down," having decreased energy, and more often than not, having no motivation. He denies other depressive symptoms but feels that these alone have been sufficient to negatively affect his marriage. Which diagnosis best fits his presentation?