In an interview with The Paris Review, Susan Sontag revealed what helped her get motivated to write:
Getting started is partly stalling, stalling by way of reading and of listening to music, which energizes me and also makes me restless. Feeling guilty about not writing. There’s a wonderful remark of Henry James: “Nothing is my last word on anything.” There’s always more to be said, more to be felt.
We're never finished, only stalling. Postponement, aka the Zeigarnik Effect is a catalyst for productivity.
A professional author may complete books but the act of writing resumes.