I can relate, it does feel like a natural physiological response where my throat locks up and my mind goes blank- not always out of overwhelming anxiety, although shyness is still there.. it's more like a habit energy of inhibition.
I occasionally try to throw in a comment here and there in groups but people look at me as if I shouldn't be speaking, as if I just said "want to see my third ear?" It's probably just paranoia but I really don't like to make a random impression.. I wish I was comfortable in a pattern of belonging where I knew my input was securely valued. We distract ourselves from keeping up with the moment when focusing on hypotheticals of what the person could be judging us for when they're the ones trying to reach out and learn about those around them (often the ones we're interested in but are too shy to approach). This focus sometimes feels as numb as an ingrained "comfort zone." I'm not always thinking about those insecurities, but they've translated into instinctual reactions.
My main fear in this context is opening my mouth and proving that I'm boring rather than remaining mysterious. How's it working for me? Not well. People can't *conclude* that you are dull and boring but they might feel insecure that you aren't talking because you don't like them. Studies show we tend to respond more positively to those who express that they like us.
I cope by standing on the periphery of groups, looking preoccupied to feel less humiliated/rejected, sticking to one on one situations, scanning for smaller groups with people I'm selectively more comfortable with. :S I can't offer any magical cure/insight but you're not alone!