Why do I stutter even though I am completely calm?

Why do I stutter even though I am completely calm? I am a 17f, diagnosed autism and anxiety. I have always had a bit of a stutter problem. However, it usually only occurs when I pronounce something wrong. I mix English and my native language Swedish up a lot. I figured that may be the factor but I’m very unsure. It just feels weird, since the stuttering usually only came out during stressful situations. Like during presentations or when being outed to speak on the spot; confrontation, etc. Now it’s just been going crazy though. I speak, and then all of a sudden a word gets mispronounced, so I try and correct it without really knowing that I am until I realize that, shit, I’m stuttering. And in the end I just give up on what I’m saying with either mumbling at the end or shutting up completely. It’s just been really stressful when I’ve come to realize that it’s become a habit. So now I try my best to avoid speaking, and if I do speak, I rehearse what I say beforehand. I’ve come to a point where I stop the rehearsal as well, since even though I am completely calm and collected, it gets messed up either way. This happens when with close friends and family as well, and none of them are really a source for me to enable the stuttering/never have been until recently. Why is it like this? And does anyone know any way to just ease it, or just any kind of speech therapy that I can go to? I’ve just grown to really dislike being silently judged for this.