Trying to find a solution to some problem is very frustrating.
"So now there are no actual problems left, but now begins the much more difficult phase. I must find some completely new ideas"
And when you try to find new ideas that you don't even necessarily need, it 1) is frustrating 2) also feels senseless.
It is very hard work. In two hours my head began to ache. So I limit the thinking "work" to 90 minutes a day.
I am already old, I am retired, why should I torture myself with such work?