When you're talking to others, you're *communicating*, in addition to just speaking.   When you talk to yourself, you already know your message well in advance - it was already in your head before you started speaking. You can say the word any way you like, distort it, sing it, and you'll still understand yourself. Even if you do stutter on a word when alone, you know your own reaction to it, and chances are you won't care and move on to the next word. Even well trained public speakers have some level of natural dysfluency, and the majority of the population don't bat an eye when it happens.   Communication is much more complex, and there are a lot more factors and perceptions involved than just speaking. You could be unsure if your message is clear to your audience. You could be negatively anticipating a person's reaction if you didn't say a word perfectly. You may worry society overall would shun you (when in reality most people don't care). Numerous factors can compound and lead to a consistent subconscious reaction. This reaction can build up to the point of entering a fight-or-flight mode designed to 'protect you' from a threat. That threat may simply be a perception of not being accepted in society, and your body may treat it no differently than being chased by a saber tooth tiger.   Check out this [treatment guide](https://understandingdysfluency.com/2018/01/12/the-omalley-stuttering-self-treatment-guide-stammering-treatment-cure-seo-specialist-how-to-guide-exercises-techniques-seo-overcome/) for a more thorough theory and explanation. The author has studied the topic extensively and provides a very modern view of stuttering, along with a 35 day treatment program. I can't comment on how effective it is (I'm on day 18, and am starting to see some results), but the program is not difficult and has some very positive elements to it, whether or not you stutter.