It is not lack of ability, but lack of motivation, lack of focus, and lack of self-confidence that prevents most aspiring authors from finishing what they started.
Stay within the confines of your chosen topic. If you start to stray away from your topic and find an urge to showcase everything that you know, resist that urge. Remember that you are writing a book, not the book.
Write a book, not the book.
Motivated authors sacrifice TV time, sleep, hobbies, and even family time.
There is no such thing as lack of time, only unclear priorities and lack of motivation. It is better to abandon a project than to work on it half-heartedly for a protracted period of time.
It is imperative that your work habits from school do not make their way into your book writing process. I am talking about the practice of typing the last words just before the deadline every time you would hand in an assignment, a paper, or even a ...
I started reading my manuscripts out loud, to hear what they sounded like. If the text flows with little effort, then I am satisfied, but if I keep stumbling and stuttering while I read, then I rewrite.