Writing the first rough draft is probably the most emotional birthing phase of the entire process of writing a nonfiction book. Once the words are on paper, you need to emotionally detach yourself from your baby. Lay it aside for a week and let it cool.
Go on to another chapter or on to another writing project. I suggest you go through this same writing process for each chapter of your book. Develop a focus, an outline, and then write the first rough draft. It might be a good idea to write the easiest chapters first if you are organized enough to determine those you can write quickly, or do them chronologically.
God inspires us to write, but He is not going to do our work for us—He will only guide us along the way. Writing the first draft is the creative part. For me, this is the easiest part. The hard part is rewriting, and rewriting, and rewriting. God never promised us that life would be easy, and He never said that writing would be easy. After 38 years I still struggle with my writing.