What little time we have is all that we have. We must still endeavor to make the world a better place so those last generations of humanity can live as happily as possible. We must also try to make our own lives, and to make each other's lives, as happy as possible. But of course, I don't mean that we should all just be selfish and try to be happy at the expense of others. That is, of course, an absurd notion, because if you do so, your happiness will not last as long.
Mutual support is what yields greater happiness.
I highly recommend the Prisoner's Dilemma sections of Hofstadter's book. I agree with the basic doctrine of Humanism in that for everything that happens to humanity, we have no one to blame but ourselves. And when we want something to change, when we want to right a wrong or better our lives, we should not pray nor appeal to anyone but ourselves. We should help ourselves and not rely on any outside influence. Humanity must help itself.