I never used to be able to appreciate why things didn't always get done. Generally speaking, if someone says they'll do something, surely there have to be some exceptional circumstances present for that task not to get done. Not so.
I always saw myself that would deliver as much as humanly possible, and ironically, I seem to be getting better at that. However, the truth of the matter is at some point there is simply too much to get done, which is why you prioritize. Taking a 1 to 4 scale (must do, should do, if there's time, oh well), I rarely see beyond level two. New stuff comes in at all levels, you always take things from the top; the tasks lower down end up being neglected for so long that they eventually get dropped to put them out of their misery.
For a long time, I found it very hard to accept living in this manner, living with an overwhelming sense of guilt. And then I started to think about a newpaper - the morning edition goes out every morning, it always has a headline, and it's always of a passable standard. Some days will be better than others, but the focus (and ultimately judgment) is always on that 24 hour period. The editor can't decide to leave half the pages blank today, in lieu of twice the content tomorrow; instead, he/she must make the best of the situation for that day and start from a clean slate the next.
Fortunately, the relief of this realization easily outweighs the lack of closure it brings. Alas, a misguided thirst for perfection is hard to quench.