Learning to say "no"

As a child, it's our favorite word. We cry and stomp our feet and scream it at the top of our lungs as soon as we learn it...much to our parents dismay. Why then, as adults, are we so afraid to use it? What is it about this word that makes it so difficult to say???
After nine days of vacation, I just want to go back on another vacation. Every five minutes or so I have another person with another issue, want or need, on top of my already huge mountain of things to do. As a sanity control method, I called my assistant and had him hold all of my calls. Not everything is a fire that needs to be put out right now. Those calls can wait. Those that show up unannounced or without appointment...sorry, I can not be disturbed right now. Please come back when you learn to call in advance. Do I feel like a big bitch doing this? Yes, in a way I do...however, isn't that part of growing up? Learning to stand up for yourself? It's OK to say no. I am still learning this after all these years. I was raised to be nice and agreeable and pleasant but if I am to honestly be queen of my castle...er office, I need to learn to grow up and assert myself. Feelings be damned!