Highly recommend The Creative Act: A Way of Being by Rick Rubin.

He has at least two beautiful chapters on this subject:

Completion p191

The Experimenter and the Finisher p205

The last sentence of Completion:

  We can't wait to finish, 
  because there's another idea calling 
  that lights us up.

The endnote for Completion:

  Is it time for the next project 
  because the clock or the calendar
  says it's time,
  or because the work itself 
  says it's time?

P.S. All his chapters are beautiful.