Help with skittish Golden Retriever?

I have a Golden Retriever who is about 2 years old. We have had her since she was a puppy, and she has always been a bit more on the shy side. I am worried because Golden Retrievers should not be so scared of the world. Loud noises, large/strange objects, small children who are too excited frighten her. (I understand the fact that small children often upset dogs because of their energy level.) She LOVES to play fetch with anything and she loves jumping in our pool. How can I help her be more ertive and less scared of the world? I love watching Cesar Milan, and I think she suffers from what he calls "Low Self Esteem". We have 3 other dogs, two Bet Hounds and a little Yorkie/Chihuahua mix, and she plays great with them, especially the little one. She's always playful but gentle with him. How can I help her?