I like trying to figure out how quotes by fiction writers apply to commercial writers. This one’s by Ursula LeGuin:

I am going to be rather hard-nosed and say that if you have to find devices to coax yourself to stay focused on writing, perhaps you should not be writing what you’re writing. And if this lack of motivation is a constant problem, perhaps writing is not your forte. I mean, what is the problem? If writing bores you, that is pretty fatal. If that is not the case, but you find that it is hard going and it just doesn’t flow, well, what did you expect? It is work; art is work.