Here are some possible motivational factors. Which one sounds about right in your case?
You are writing your book to earn money?
If your writing revolves around making a living, you may want to diversify: write some magazine articles, take on some write-for-hire projects, do some editing, etc. I don’t mean to be a wet blanket, but earning a living from writing books takes time. You must have enough money to get by during the time you are writing, editing, and submitting the book as well as during the first 6 months to a year after publication while you wait for the proceeds from book sales to come in. It’s wonderful, but definitely a fluke when a writer receives a large advance on their first book. We often hear about people when they get that big break. Of those people we consider to be overnight successes most of them had probably been quietly writing in obscurity for ten or more years prior to that.
So, it’s fine to write for money, just make sure you are realistic about it and have some income to get by on until your book money starts coming in.
You are writing to help others.
If you are writing to help others, you may or may not care about money. What’s important to you is to publish your views on a particular topic or to publish some vital instructional manual that will help others.
Also, many of us begin writing to help ourselves. We start out writing a certain book just to add a publication to our resumes in order to build a reputation for future publications. If this is the case, get started and use all your spare time to accomplish your goal.
Writing to get the word out.
Many people know a great deal about a specific topic and feel compelled to share that information with others. Their motives are honorable and the information they share often helps others and can not be found elsewhere. This is another case where you may be doing it for the love of the topic. If so, get started and write that book. If you need the compensation, then start by trying to find backing for your project through those (publishers or otherwise) who would be naturally interested in your topic.
Writing for others.
Another reason for writing is to save information for others. Many people write memoirs or family histories to ensure their family story is recorded for coming generations. These are not usually money makers, but will require a period of research to gather the information from all the branches of the family tree.
Writing for the love of writing
Many people write just because they love wiring. If this is the case, there’s no way to avoid it. You have the passion, so go with it.
Many people write for a variety of reasons. Knowing those reasons might clarify your goals for you and help you stay motivated. Take some time before you write to pinpoint the main reason and a couple of secondary reasons that you want to write. It’ll help when you stumble on one of those dry spells when the words don’t flow as easily. Putting your focus on why you want to finish your project will help motivate you through the slow periods.