In my opinion it sounds like you are tense most of the time. Just the idea of speaking gets you so tense that you can’t get a word out even when you are alone. There is no outside tension sitting alone in your room. The tension is coming from inside you. I know this may not help but you need to learn to chill. My suggestion would be to find someone to help you lower your tension. Speech therapy may not be helpful when you are at this level of tension.