A book is not a thing of one sitting, like a poem, but a longish thing which takes time and energy, and since it takes skill, too, the first effort or maybe the second may not find a market. A writer should not think he is bad, or finished, if this happens, and of course writers with real drive will not. Every failure teaches something. You should have the feeling, as every experienced writer has, that there are more ideas where that one came from, more strength where the first strength came from, and that you are inexhaustible as long as you are alive. This requires an optimistic turn of mind, to say the least, and if you don’t have it by nature, it has to be created artificially. You have to talk yourself into it sometimes.
From Patricia Highsmith, Plotting and Writing Suspense Fiction (1983).