Sometimes when you suddenly find yourself in a dark mood, you just don't feel like doing a dumb little exercise to get out of it. Doesn't matter whether or not the dumb mind exercise would work or not. If you don't feel like doing it, and haven't committed to doing one anyway, you won't do one. Okay. Then instead, do something difficult. Do a puzzle, or learn something difficult like playing the piano or the guitar and then practive like mad.
Tackle some difficult home repair. Work out until you are tired. When you do something difficult, it is necessary to bring your full concentration to what you are doing. Which means you have to withdraw your attention from concentrating on your dark mood. Dark moods cannot think themselves. They are completely dependent on your concentrating on them and thinking them to the exclusion of everything else.
The only way out is to concentrate on something else. If it is something difficult, that is even better.