Hey there. I do not stutter when alone but I had done it some times on my lifetime. When the thought or situation becomes so vivid is like the real thing is happening and I block. I used to block on feared words too when alone as a kid. They were just to emotional dense for me. You said you stutter no mather what but I´m sure that there are degrees of your stuttering intensity depending the situation or how you feel that very day. Maybe is a good start to noticing those situation which you are more fluent and go from there. Explore those feeling, write down your emotions and thoughts. From there you can try to take that fluency to the more dificult situations. Step by step. Take care