How can I stop talking to myself?

Seems like when I'm alone and I'm thinking of something that might be worth saying to somebody in a conversation, like a little ancedote about something, I dont realize it but I start saying that thing out loud to nobody in particular...almost like I'm rehearsing saying it or speaking to somebody. I've always been nervous public speaking; is that my subconscious way of practicing it? And how can I stop doing it?