I think I know what to do, since I had a similar problem. When I fixed it, I even became more fluent when I actually speak. Hope that helps you. Personally, I think in conversations. I usually imagine as if I am speaking to someone or even give a presentation. When I do that I noticed, that I unconsciously move my speech muscles (lips, tongue etc.). Even though I do not actually make sounds, I engage my speech muscles, which makes me “stutter in my thoughts a bit”. Tongue is especially the problem. So what I do now is I try to consciously relax my tongue and not move any muscles of the face. It would break your brain a bit at first, but you will get used to it. Just practice (if you have the same problem of course). This way you will not stutter in the mind. By the way, this will also improve your speech, since if you speak a lot it your mind, this might reprogram it a little to forget that you stutter. At least it helped me and I hope it helps you as well. Let me know how it goes and good luck.