On his 31st birthday, Meriwether Lewis (of Lewis and Clark), in the midst of the expedition he led, which opened up the western frontier to the United States, wrote a journal entry lamenting that his life was half over and he had nothing to show for it. I also never knew that he was bi-polar and wound up committing suicide a few years after returning from the expedition.
“This day I completed my thirty first year, and conceived that I had in all human probability now existed about half the period which I am to remain in this Sublunary world. I reflected that I had as yet done but little, very little indeed, to further the hapiness of the human race, or to advance the information of the succeeding generation. I viewed with regret the many hours I have spent in indolence, and now soarly feel the want of that information which those hours would have given me had they been judiciously expended. but since they are past and cannot be recalled, I dash from me the gloomy thought and resolved in future, to redouble my exertions and at least indeavour to promote those two primary objects of human existance, by giving them the aid of that portion of talents which nature and fortune have bestoed on me; or in future, to live for mankind, as I have heretofore lived for myself.”
– from Lewis-Clark.org