It always seems to crash down harder when you try making goals to get things straight. The important thing, I think, is to keep trying. We don't have to live with a false sense of optimism - We just have to believe that even the smallest efforts will make a difference, even if it's only in our own lives.
Honestly, I find my low points the most interesting times. Sure, it's nice being on an upswing and being ridiculously happy for no reason, but darkness creates such a stark contrast to the things around me. This becomes even more stark at work - One moment, I see a parent playing with his or her child, the next, an old man with no legs wandering the aisles in an electric cart. Compare both to me, where I'm sullen and moody, feeling like my efforts are meaningless and I should just pretend to be sick to go home - A future full of potential, a past full of painful accomplishment, and a present with neither.*
Sadness is a heavy burden, but it teaches us more about ourselves and the world than any other emotion. It also makes you feel rather poetic. Considering that a number of artists and even a few charismatic leaders suffered from melancholia, these thoughts - That sadness creates wisdom and creativity - Doesn't come too much as a surprise.
*From my warped, manic depressant viewpoint. I know I can get somewhere in life, but it's hard to see through the haze when it gets so thick.