Another source of stress is: Striving to be somewhere else. Whenever you take a shower, or eat, or lie in bed, and you feel that you ought to be somewhere else (working) that's stress. That not only applies to whereabouts, but also to situations like "I should be ready with X so I can take on Y". To reduce that, you have to establish a clear declaration of working and resting time - resting means really switching off, not allowing thoughts to pull you back into work mode.
EDIT: I think the word I would use for countering this type of stress is "dedication".
Exactly right! If we don't 'live in the moment,' focusing on whatever we are doing, that is stressful and increases unhappiness. Pardon a cliche, but it is so very correct: people who live in the past tend to be depressed, people who live in the future tend to be anxious, and people are generally happy when they are absorbed in their current activity.
I used to get this "should be/doing somewhere/something else" feeling a lot while browsing Facebook. I would see peoples updates and think I was slacking, or not seizing my day or whatever. That feeling quickly disappeared when I stopped using Facebook. I eliminated that external "motivation" of others updates and focused on where I was and what I was doing at that moment, not what I could be doing.
Definitely, I think that striving to be somewhere else or just the idea of not having everything done for something that you are dedicated to can be a source of stress. A clear plan for when and how you are going to handle your Todo's can help relieve some of this stress.
EDIT: I think the word I would use for countering this type of stress is "dedication".