Never be afraid to offer a smile; sure the risk is that a few foolish people may misinterpret your kindness as weakness, but the sweet reward is that as you make new friends and encourage others, the foolish will learn the error of their ways because you did at least show them your teeth.
Think of the most discouraging thing that you are currently carrying or the biggest project you can’t seem to tackle. Forget about how it happened, forget about how you got there, just look at the clock, and before that number changes, do one little thing to make it better. After you do, enjoy your moment of victory, and then do it again. In less than the time it takes you to blink, the past will burden you no more.