You Are Not Good. And Your Mom is Not Good.And this, people, is why most commoners are described as "neutral" or "lawful neutral" or "chaotic neutral." Nitpicking about the alignment system aside (and someone will post with it, so I'm acknowledging you in advance), this says something about people in general that sometimes needs saying. Everybody likes to think they're doing their best, and everyone wants to think this makes them an okay person.
"I have made mistakes in my life, but basically I think I'm a good person."
I'm sorry, but you are not a Good person. You go through your life, you don't stab anyone in the face, you don't break any laws, you don't take pictures of naked children, and… so what? You want a medal for that? Shut up.
The sad fact of the matter is that if you aren't exerting yourself for a cause, if you aren't exerting yourself for something, you aren't Good. You probably aren't Evil, but seriously: get over yourself. Before you can really get into the mind of a Good character you honestly have to come to terms with the fact that you, as a person, are probably Neutral. Your character is a much better person than you are.
The reverse is also true for villains, and should come as no surprise to people who play Evil characters, since most people don't consider themselves Evil. Characters are generally much more than the players who play them. Villains are blacker, heroes are nobler, and when you play one of those characters you should come to terms with that. Even though it probably hurts you a little bit to contemplate it, if you're going to even try to play a Good character you need to play them as a much better person than you personally are.
But that's a different kind of "good" person than the kind of person who's risking something important for the sake of the prodding of their consciences. An abortion clinic bomber has more reason to believe in his/her D&D-classified "goodness" than most people, whether most people would agree or not. At least they've decided what they believe and are doing something about it, even if I occasionally wish an adventuring party would storm their evil black towers of misogynist self-righteous dickery.
To put it Walter's way, "Say what you will about the tenets of National Socialism, but at least it's an ethos."