I'm actually very similar to this as well. I have a very loud older sister and my whole childhood she just took it upon herself to speak for me, and she still does. As a result of that I just sort of stopped talking most of the time. As far as social situations go, I'm usually okay with people my own age. If I am hanging out in a big group of friends I can chat with them all easily. Other times however I find it much harder. For example, if I am out for dinner or at a BBQ with a group of my Mum's friends then I find it hard to speak. It's not that I don't have anything to say, it's just that I can't ever seem to find the time to say it.
They all just seem to talk non stop, and without straight out interrupting them which I don't want to do, I find it very hard to cut in and say something. Because of that I just often don't say anything at all, so many people know me as 'the quiet girl' now. My Mum has even told me off for being rude and not talking before, but she doesn't understand that I want to talk, but just can't find the right way to do so.
This probably explains why so many of us spend this time online, while we are typing people listen to us... well "read" us at least.