The reason is its not because you don't know what to say, it's because you think what you want to say isn't good enough. Whenever people lock up its because they hold back things because they fear rejection. Eg around your friend you can talk but not others you don't know so well, this means there's something you want from the others, that's preventing you from expressing yourself. Is it security? To make sure you're not made fun of or ridiculed is that why you don't talk. Is it approval? You don't talk because you don't want to say something that makes them dissaproval of you? I reccomend a book called loving what is by Byron Katie, if you discover the reason you lock up, you can then use that book to release it.
