"I should" + "I didn't" = stress

We clutter our minds with vague promises about what we should do, what we could do. But there is always more to do than there is time to do it. Most of the stress that people feel doesn’t come from having too much to do - it comes from not keeping agreements they’ve made with themselves. When you tell yourself you ought to do something and then don’t do it, you experience self-doubt and frustration. You can fool all of the people some of the time, but you can’t fool yourself for a second.

…if you allow too many things that represent undecided, untracked, unmanaged agreements with yourself and with others to gather in your personal space — that will start to weigh on you. It will dull your effectiveness. You’ve got to dig into the mess and put those things to rest.

- David Allen in Fast Company: [interview 1] [interview 2]