I'm exactly like this. It's pretty rare that I shout up about anything, even when it's pretty obvious that I'd do well to break silence. Strangely enough I remember being pretty vocal about everything when I still lived with my parents, though we were all being quite negative to each other at the time, and my words were no exception. The negativity had to go, but somehow I lost much of my ability to spontaneously say anything very much.
I'm not sure what part shyness plays in it. I've always described myself as basically a shy person but I've certainly had my moments. I suppose a lot of it is a distrust of being spontaneous, knowing by experience how much damage I can do by talking without thinking.
Sometimes, when I'm in the right mood and conditions are right, I just bulldoze through it, and then I can get almost bubbly. Not long ago I engaged in a very rapid-fire, vocal conversation with somebody I knew vaguely. On another occasion we suddenly shared a silly joke together and really made each other laugh. But beyond those two events, we're very shy of each other and barely know what to say. I guess for me it's like pearl diving - I can hold my breath for a short time, and at the time it will feel for all the world as if I could go on forever, but I always find myself coming back up for air and staying there for most of the time.
I think it's a thing that can be worked on, but in my case it's definitely not just a matter of breaking the ice once and then everything is easy. I would think that if anybody with this shyness problem (or whatever it is) became deeply interested in doing better, then it would be possible to make some improvements. I'm quite a negatively-driven person, so it helps me to stay mindful of the idea that if I keep silent, then I'm denying a useful resource to my friends, which is probably more harmful than blurting out the occasional stupid comment along with useful opinion and truth.