"The solution is to be experimental on a personal level: find a way to measure your productivity, even if subjectively, vary your working conditions in systematic way, and see what makes a difference for you."

Exactly. As a working, self-supporting artist, I learned pretty early that I could rely on certain techniques for working through problem times. The tools of the job delighted me, drew me back to the work table. I learned what it takes to move around problems, see them from a variety of perspectives. It became important (if frustrating) to junk stuff that was bad and that I couldn't fix. I learned that walking out the door and doing something completely different helped a lot. Most of all (and this took ages) I figured out what makes my brain work -- and that's when I began to develop some serious self-respect! Maybe thinking in terms of keeping your creativity alive, rather than "getting the job done," would help. Even artists stick to work schedules: self-discipline (and/or disregarding the demands of others) may sound like a bummer, but it's the oil in the engine.