I've had the same problem when I'm confronted by an authority figure. There's a lot of nervousness that can kick in when I sort of feel I'm being asked to defend myself, even though usually they're just trying to get stuff done efficiently. When I am in those situations, or am going to be coming into them, I tend to think about things in terms of chess moves... and that's probably not the most constructive way to approach those kinds of conversations. Indeed, I think it's unwise to have fresh in mind the idea of responding in a *big* way - considering large issues and swathes of the sentiment between myself and the other person... when actually, it's a really small and unimportant interaction that's going to be had in the kitchen. Maybe, you could try this: Sit down and think about these types of situations where you've been blocking with your speech... and rather than trying to rehearse the wording for hypothetical responses, think about the kinds of realizations that might need to come to mind that will help you deal with that moment of interaction, deftly and constructively.