If rewards alone don’t keep you writing steadily, try to set up your worktime so that you have a small victory every day.
For longer documents you can think about D-days for each part of your document.
Need even more structure? Set a “mini D-day” for each time you work. Pick a section or even a part of a section of your writing that you will finish it in that session.
Even a tiny bit of progress is better than no progress at all.
Whatever gets you closer to D-day.