happens too often. Most of the time I stay quiet because 1. I'm not admitting that I am quiet, because I'm really not, I just don't know you 2. Some unintelligent thing will come out of my mouth if I open it and 3. What's the point in telling them why I'm quiet when they wouldn't understand or care or stick around long enough anyways
This website has helpful information on how to change the way you think in social situations. Scroll down to the "Challenging Unhelpful Thoughts" section of the document and you will find 2 exercises that you can print and fill out that will help you to recognize the thoughts that are not helpful when you feel anxious.