Same here (18f) I still stutter when I’m alone too, except I’m completely smooth when I sing. I’ve done years of music lessons and tutoring for my voice and instruments and that’s my outlet for when I need a break from stuttering. I also find if I know that I’m going to stutter on a word (I can legit feel it in my throat), I try to describe it and pretend that I forgot what the word is, then the person begins to guess and once I hear them say it fluently, my brain automatically can replicate it and I can say it smooth. It’s so weird but fascinating. Im pretty sure it’s like a communal unison thing where by hearing others speak fluently, the brain recognises it and replicates it. There’s some technology around that tries to replicate it actually.