I do stutter even when I'm alone

I do stutter even when I'm alone I heard that most of stutters don't stutter while they're talking to themselves. But it doesn't make a change with me. I still stutter when I'm alone. I have problem with most of the letters but especially with letters k, g, d, p. Some says stuttering is mostly a psychological problem but I think my stutter is not psychological at all. What do you think? Maybe its reason is about my brain, face muscles or something.