I don't think I've ever stuttered in my head. I thought that was one of those rules. Like how you can't stutter while you sing. It might be a mental block you have to push past. Force yourself to speak out loud more and work on the words you can't say. I had a hard time with saying words that start with H, but after forcing myself to answer the phone all the time I am waaay better. Good luck to you though man