I have this problem, too, although not as badly as I did about a year ago. What would happen is someone would make a stupid or insensitive comment, and I wouldn't retort because I would be trying to figure out why they'd said it! This is the SA talking...my default assumption was that I'd done something to provoke them, and that lashing out would just make me look like a jerk!
I don't think it's always necessary to stand up and respond to such asinine behaviour...a better way might simply be learning not to take it seriously. If you can believe in your self-worth enough not to let such hostility get to you, you'll communicate that in the way you respond to it: the way you laugh, roll your eyes, and walk away. And sometimes when a bully sees his/her attack butted away so casually, it's every bit as biting as the wittiest comeback!