I'm hitting the big Four-Oh in a couple of months with some seriously mixed feelings.

On one hand, I don't feel 40, or middle aged, or whatever (except that I have less tolerance for LOUD noises and bad behavior). I'm in better shape than I was at 30, at the same weight I was at 20 and have been told I look much younger than my age. I have a successful career, great friends and a wonderful, supportive family. I have never suffered from health problems, and neither has any of my family. I consider myself a truly lucky person.

Turning 30 never bothered me... so why I am so upset at turning 40?
Does being single have anything to do with it?
The thought of my next birthday makes me sad, for some reason.