If you don’t pay your rent or mortgage, someone is likely to scream at you. If you don’t take out the trash, your house will quickly smell atrocious. But nothing really bad will happen if you don’t finish your book. No one will scream at you. No one will kick you out on the street. And that’s why so many people sit on their book ideas for years.
How do you actually finish a book? Write like a bulldog. Give yourself a set deadline for when a draft needs to be completed; set aside a given amount of time each day to write. And then protect that time like a bulldog. Unless it’s a life threatening event, you write and write and write until you reach your goal.