Writing gets accomplished in different ways. As long as the words end up on the page, it doesn't matter how they get there. At least that's the conventional wisdom. However, what I've found is that some ways of getting words there are more efficient, less painful, and more fun than other ways.
Not every writer will agree with what I consider the ideal method. Mystery writer Lawrence Block, for instance, used to think about a book for a long time and then, when ready to write it, hole up in a hotel room and produce the thing by working day and night until it was done. Then more months of thinking but no writing until the next one. Hey -- it worked for him. But even he admits it's an exhausting and grueling form of working method.
Here is what works for me and what I recommend you at least try if you're groping around, trying to either get started on or back to a novel:
-- Write at the same times each week. Every day if you can manage it, a minimum of three times a week if you cannot, but at set times that you set aside as sacred to writing. This prepares your unconscious to produce words at that time. I write six days a week in the morning.
-- Work with your biological rhythm. If you're a night person, work then. If you're best in the morning, get up very early and write before you go off to your day job (Gene Wolfe did this for over twenty years). Your fiction deserves you at your best, whenever that is.
-- Don't write too much at one sitting. Of I go over 2,000 words, I find I have more trouble producing the next day. It's as if I then need longer to recharge the battery. When I began, I wrote four long-hand pages a day (about 850 words). If that's too much, make it 500.
-- Stop when you know what happens next in the story. It helps to get back into it the next day if you are in the middle of a scene or a clear sequence of scenes.
-- Use getting-started rituals, but do NOT make them lengthy. I used to do the NEW YORK TIMES crossword puzzle to "get warmed up." When I noticed that on Saturdays this was taking me two hours, I stopped. Now I have one cup of coffee, check but do not yet answer my email, play three games of solitaire, and sit down to write with the second cup of coffee at my side. That's the warm-up ritual.
However, you do it, writing may sometimes seem difficult but it shouldn't consistently seem like actual torture. Discipline can help you find your natural rhythm, rather than have to impose it each day by a wrenching act of will. That's what dieting is for.