Its not a character flaw, or learned behavior, its a biological feature. A feature that is often initiated by some stressor that puts a person into this oscillation of moods. I can say that if I had seen a psychiatrist at the time of that initial stressor for myself, instead of basically hiding in my apartment, a good chunk of my adult life would have been a lot more pleasant.

As to the diagnosis... mania + hospital... there aren't a lot of things to pick from to put in that box.