If you wait till you’re in the mood to write, or the mood to play music, or the mood to exercise you’re leaving a lot up to chance.
Because you don’t know how long you’re going to be waiting for the mood to come back. Plus, you have the added layer that once you start getting to the end of creating something you care about the fear starts to creep in, and fear can be far more powerful than the mood to do something and so you never quite get around to finishing.
But if you decide, say, that every night at 7 pm you write for an hour then regardless of whether or not the mood has struck you you’ll get something done. And the fear might be there telling you that you have writer’s block, but it doesn’t matter because you’re sitting down to write.
And through enough persistence, you’ll find that the right mood does start coming when you need it, and it will come at 6:55 pm.