I always underestimate how long things are going to take. Either I'm an incredible optimist, or I'm an extremely poor judge of time. I think it's actually a combination of the two.
I prefer to think of myself as an optimist, but when things take longer than I think they're going to - as they invariably do - I feel my hopes crash and my optimism dissipate. Then, I berate myself for my inability to be more realistic in my estimates. This does no good, of course, nor does it prepare me to come up with more accurate estimations the next time around.
I've read a lot about time management, and estimating time. I've read tips like doubling the time you expect things to take, or adding 15 minutes, or some other such manipulation of the numbers, but I've still not found a reliable formula for estimating how long things will actually take in my own corner of the real world.
Truth be told, I haven't really tried. Piling that much reality on top of my already overcrowded to-do list might deal the fatal blow to my optimism.
So I continue to delude myself, thinking I can accomplish so much more than is humanly possible in the time I have available to do it. I guess it's not really the end of the world when things take longer. Eventually they all get done. Mostly. The important stuff anyway.