You're not alone, that's for sure. I'm in the exact same boat where my mind starts racing when I know I have to speak. In my first speech therapy session, she tried a few techniques on me. The first one was massaging the vocal chords. Think of it as if you swallowed a pill and it's stuck in your throat. Using your thumb and index finger, press gently against your neck (around the throat) and start from the top near your jaw and stroke down to where your neck ends. While doing this a few times, breathe in through your nose and exhale through your mouth in the same way you would blow out a candle. It is important that you imagine blowing out a candle slowly. Try to extend the exhale a bit and you can even make a quiet humming noise while you do it. I think this kinda warms up and loosens our vocal chords/neck muscles that tighten up during a block. Give it a shot because what do we have to lose?