If you frequently abandon writing projects, we highly recommend Duckworth's Grit, which she defines as:
Duckworth writes that grit is a better indicator of success than "talent" or IQ scores.
We culled from Grit that the success of one book shouldn't be your top-level goal, because you may not be able to sustain grit if that book isn't "successful". Instead, your top-level goal should be to be a New York Times bestselling author or, even better, something altruistic like disclosing nympholepsy in popular culture, which you may attempt to accomplish through writing books, blog posts or (online) magazine pieces. Therefore, being a successful writing may serve better as a low-level or mid-level goal.
Here's an animated summary of the book until you can get a copy: