I don't know if this is any help to you or your story but; I stutter when talking to myself when by myself. It's incredibly frustrating. I'm 43 and have had the gift since I was 5 or so. My dad had it too.  I don't think it's anything dealing with being socially anxious with me personally. It's something broken in my brain. Some days are better than others. Some are worse.  Idk if this helps you with your character at all but I just wanted to say it's not all about social anxiety(not saying you made that claim because you didnt) but sometimes it's just for whatever reasons this is just something we do because that's how we are. It sucks lol. Good luck with your story.