-Somerset Maugham
In strong agreement with this comment. The idea of writing every day is to create a habit of writing. It is not about motivation. Imagine taking a walk after dinner everyday for many years. It becomes a necessity. Similarly with writing, you want to convert a conscious decision to work, into a visceral feeling that you just have to do it. Consequently, you don't have to dip into your limited willpower supplies.
Source: I just finished writing a book after writing daily for 2+ years. Sitting down to write became much easier when it became a habit.
Or, enough discipline to get started: http://www.paulgraham.com/hs.html. I also like the Seinfeld "Never break the chain" method: https://www.writersstore.com/dont-break-the-chain-jerry-sein... (if "method" is not too strong a word here).
For the rest of us, who aren't so lucky, discipline will have to do. Writing every day (or every two days, or every week, etc) isn't a about finishing a project or being the best you can be. It's about making writing a permanent fixture in your life. The same goes for any other skill.
People imagine the quality gap between things created in bursts of highly motivated inspiration and things created through slow, at times plodding, hard work is high. It isn't. If it were, there would be markedly fewer books, games, movies, etc. out there. By and large, if you're good at something, you'll still be good at it even if you don't want to be doing it. You might not be as amazing at it, but you will still have the skill. That's what practice is for.