>>I've had so many experiences like this.... but I tend to not say anything. I'm always >>worried about sounding stupid.

How about... going to a place where nobody knows you and start talking to strangers there at a store etc. Nobody will ever remember you the following day if you think you said something stupid. But it's a practice, the more you practice the less worried you become and if you worry less, you will be more confident which means you will not sound stupid.