They can't. If you're rational, empathic and honest, you're going to be upset at the state of the world. You can try distract yourself, or make small impacts, but you can't make a significant difference.
I still have people to love and things to appreciate. I know that civilization is pretty fucked right now, and I can't say for sure there's an other side to this tunnel we're barreling down, but that makes it all the more imperative that I don't take the good for granted. Despair and happiness are choices.