I struggle with the same thing. I try to think that people dont care about others as much as we think we do. I mean consider that you see someone dumb do you think of them often? People forget your dumb moments because everyone is fixated on themselves. I also struggle with thinking in such situations my thoughts get muddled so i try my best to focus on what i should say instead of the stutter. Take pauses and let yourself think properly. Rushing into it ends up in us giving dumb answers. No answer is better than a dumb one so take your time and articulate slowly. People often like slow talkers