First, remember that RIGHT NOW is the only time you really have; even if you tell yourself that you will finish your book at a later date, you have no real way of knowing if that time will be as busy and stressful as life is right now. You aren’t there; you can’t predict what will happen in the future. All you have is right now.
Secondly, remember that you can only write (depending on how quickly you type) about three words right at once. Unless you’re a pecker-typer, then you can write one. The point is, pressuring yourself to write ALL THE WORDS at once is unrealistic, and literally impossible.
Now that you have remembered these two things (you only have NOW, and you can only write a few words NOW), take advantage of spare moments.
We all end up standing and waiting far more than we would like. Lines at appointments, being on hold on the phone, waiting in a meeting for the last person to arrive, or standing and waiting for your cab/bus/ride, when put all together, compose hundreds of minutes over the course of a week.
Take back this time. You would be absolutely shocked at how many words you can write when you only think about doing three or four words. Writer’s block generally only strikes authors who are thinking about the last page; think about not only this page, but this very paragraph, and even more than that, this very phrase, and the words will come to you.
Write on your phone. Write on a folded bit of paper you carry in your pocket (I’ve done this). Write in a small diary. Use the wasted bits of time in your life TODAY to write down only a FEW words at a time, and I promise that your novel will march to a satisfactory conclusion.
Then you can write your next book.