"Shyness and introversion are not the same thing. Shy people fear negative judgment, while introverts simply prefer less stimulation; shyness is inherently painful, and introversion is not. But in a society that prizes the bold and the outspoken, both are perceived as disadvantages."

This. So, very much. More people need to understand this concept. I grew up being both introverted and shy, but the wanting me time wasn't because I viewed my own actions around folks as painful/embarrassing/etc, but because it was overwhelming. The shyness was separate, and IMO, something that I've gotten better with and worked to overcome. I can't help but be overstimulated, it just happens.

A friend described the difference 'tween extroverts and introverts in a great way. Extroverts charge by spending there time with lots of people, introverts at a party, will get their energy zapped. However, when spending time with themselves, introverts are able to recharge, where as extroverts cannot.