I do that on occasion, but not to the same degree as you. Also, when I first wake up in the morning for school and am really tired random sentences start running through my head that make no sense and might even have words that I've never heard before or don't exist. It's like a word salad. It's almost like I don't have any control over the sentences that I am thinking since I'm so tired. Almost like I'm just sitting there listening.I don't really talk to myself, but I do ponder a lot(especially late at night when I'm lying in my bed) about conversations I might have with people. I'll think of how to say my sentences the best as possible, then I'll predict what the other person says and think of a retalliation again. Or when I'm not sure I just think of multiple answers. Also, when I have got a story to tell, I'll prepare it so that when I tell it to someone, it'll sound exciting and not dull. So probably, if I ever tell you a story, I have thought of how I would tell you that story beforehand. Just like what I'm telling right now. There have been countless nights where I've thought about how I would tell this to people. Even that last sentence about how there have been countless nights of thinking.
So yeah, this doesn't have much to do with your problem, but I just wanted to tell that I also have conversations in my head. Only I don't have them with myself, but with 'imaginary' real people I know.