I use to have that problem all the time in my therapy sessions. I literally could not speak, just had conversations happening in my head that couldn't find their way out.

Can you practice talking to yourself out loud when you are with yourself? It's like the Tin Man, we rust shut I think. Practice saying out loud what you think in your head occasionally, comment to yourself about the sound of your voice, any dumb thing that springs into your mind. That will lubricate the connection a little and be a voice you're responding to (nevermind that it's your own :-)