When you're bummed out, I bet you sometimes mope. Moping is something people do when they're feeling gloomy and doomy and sad.
Moping never solved a problem, but we all do it when things aren't going our way. A moping person is usually frowning, slouched over, quiet, and maybe even lying down. We mope when we're sad, but part of moping is directed at other people, because moping is at least a little public. When someone says you're "moping around," they can tell you're trying to let everyone know how sad you are. To mope is to say "Hey! Somebody cheer me up!"