Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.
Don't be ashamed to admit that you aren't a good one. Many people are not, and I'm definitely not as good as I should be, given my profession. But then again, in my profession, most people are terrible estimators. That's why I picked up the latest book from Steve McConnell, called Software Estimation - Demystifying the Black Art. This is the same guy who wrote Code Complete, which is considered by many (including me) to be the ultimate manual on practical software engineering. I have a review of his new book on BlogCritics. Check it out. This should be on your book shelf, right next to The Mythical Man Month.