This was me for so many years and I never knew why until a year or two ago. When I would be around others I would be subconscious about my speech but while alone it ever happened and why is that? Because alone we don’t feel pressure to speak. You should Defintely read a book called stop stuttering short course: how to break the stuttering code. Heres a free sample pdf of the book that I found here the other day- https://www.stopstutter.org/get-free-book