Question: Greta van Fleet seeks therapy because she is feeling sad. Greta says she has often had periods of extreme sadness in the past and they typically last between 6 and 8 months. During those times she overeats, has trouble sleeping, feels exhausted all the time, and thinks a lot about dying. At other times, however, Greta says she feels wonderful. During these periods of feeling wonderful (that last about a week or two), she gets a lot done, feels as if she could do anything, talks a lot more, gets little sleep, but doesn't feel tired. Greta says she loves her "up" times. These periods appear to never cause her any trouble, and she is able to function normally at work and at home. Greta's most likely diagnosis is: