Theorizing massive improvement.

[kerm]

For example, my last image was done linelessly and I was going to color it with a combination of gradient and coloring, but I got tired by the time I did the gradient.
In any case, I'll go back to my other drawings and write down what I need to work on and try to remember to use the Pomodoro Method of 25 minutes of work and 5 minutes of break so I don't get the symptoms of "Fuck it, it's done."

For me, the moments when I quit working are connected with dissatisfaction. I might be obvious, but doing smaller, faster things before starting a big, time consuming image is a good idea. If the effect is bad, whatever... you didn't waste days on one thing. If you gain confidence with something you try on a smaller scale, it's a good time to start bigger piece of art.
The Pomodoro Method is OK, but I think 25 minutes of work is not enough. They say that "flow" state starts after 20 minutes. In the "flow" mode you gain proper focus, little things don't distract you and your brain stops telling you to check facebook etc. :)
I think that 1,5 hour - 2 hour session of work and then 20 minute break is a good idea.
Remember not to get distracted in the first 20 minutes. After that, if you get into "flow" mode, concentration is waaay easier.
But that's how it works for me. Maybe you should experiment and find your own way.