Trouble comes in many forms. It's felt both internally and externally. Some troubles are easier to resolve than others. Pop psychology tells us not to worry about what we can't control, but we do.
We worry about the economy, the environment and global warming, and we fret over national and international violence. But really, what can we do about such things?
This psalm gives those who trust in God three assurances in the face of such worries beyond our control.