If you're completely fluent when speaking alone, that means your stuttering is not entirely physiological. If someone has true stuttering, they will not be able to speak completely fluently even when speaking alone without any kind of social pressure. In my case, I cannot pronounce certain words even when speaking alone. I always block on initial consonant sounds, and need to artificially add vowel sounds at the beginning. This is the same whether I am speaking to myself or speaking to a large group of people.