From personal experience. (Sobs quietly over her smoothie.)
1. Re-start it once a year for three years.
2. Decide to switch main characters more than once.
3. Write it all on your phone and lose a significant chunk when your toddler gets the app open.
4. Get distracted by new shiny ide- squirrel!
5. Have a baby. (Actually this one I recommend. It just delays the writing a TINY bit.)
6. Tell your sister or your best friend the whole idea and lose all motivation.
7. Swing wildly between manic confidence and utter revulsion in what you’ve written.
8. Take three years to write it.
9. Get feedback on early chapters and lose all motivation.
And 10. Blog about writing instead of actually WRITING! 🙂