Begin at the beginning, middle at the middle, and end at the end. Writing is a process. Despite the need for creativity, the actual production of your book is a method which you need to work through. Silence the distractions and produce your word count each day. Silence the artist in you and grind out the content.
Not all artists work this way.
Many painters, and there are many examples of the old masters doing this, work on several canvases at once. But they don’t stop there. They still sketch and work on new ideas.
Whilst Claude Monet was painting in London in 1900, he wrote to his wife telling her that he had 65 canvases with colour on them, and he still needed more.
It is perfectly okay for writers to work in the same way.
Deadlines are deadlines and the production needs to get done, but you will still get new ideas so don’t ignore them.
Write them down. Run with the idea. Plan it out.
Write your structure notes. Maybe even write the odd scene or two.
Artists know it is okay to have several canvases on the go at anyone time.
Writers are artists too.