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How can I improve my estimates?

I've found that the more detailed my estimation process, the less optimistic my estimates are. Generally speaking, if I break down what I'm planning on doing into units which I think should take a month each, I end up with an estimate which is 2x less than the actual time I will take; if I break things down into units of a week each, I end up with an estimate which is optimistic by a factor of 1.5x; but if I break things down into units which I think can be done in a day each, I tend to come very close to being correct.

I don't know if this trend is something other people experience; but if it is, the obvious solution is to think more carefully about all of the small steps which make up the big steps.