On days that I get down about the problems in the world, I try to remember that I make people happy for a living. Even if the happiness is only for a few minutes or hours, it's worth it, it has value. I also remember that the donations that I give in support of material matters (water, food, shelter) come directly from people that I have, hopefully, made happy.
On days that I'm down, I try to remember that there are positive circles, positive cycles. I can then turn towards the negatives with renewed hope and vigor.