"You must always work not just within but below your means. If you can handle three elements, handle only two. If you can handle ten, then handle five. In that way the ones you do handle, you handle with more ease, more mastery and you create a feeling of strength in reserve."
"We can't have it all, and worse yet the desire to have it all and the illusion that we can is one of the principal sources of torture of modern (...) free and autonomous thinkers."
The reason for the "torture" is simple. You think you can handle many tasks, you want to handle more tasks, therefore you suppress the obvious: each new task doubles your non-value added time spent on switching contexts (remembering the goals, the last actions, the next action, the players, etc.) A classic conflict between expectations and reality.
So relax, focus on what's important, and remember that it's better to do three tasks thoroughly than ten tasks poorly.
And to help you relax, here's an excellent podcasts from Barry Schwartz on why More is Less