Why do I only have a speech impediment/stutter only around people I know? (sorry if wrong sub)

Why do I only have a speech impediment/stutter only around people I know? (sorry if wrong sub) I'm in tenth grade, I find it hard to speak coherently to people I know, like my classmates all the way to my parents. I find it hard to regulate my air when I talk to people (or smth like that), it's like I run out of air so I have to compensate by talking fast therefore incoherently and I sound like an absolute idiot. My mom says I talk like I have a bunch of plums in my mouth. Words also come hard to me, I forget a word every 3 words, I sound like an stupid 5 year old IT'S HORRIBLE, this is why I mostly don't talk to my classmates I try to make an effort to talk clearly with my parents and classmates but I still find it so hard. My parents just say I need to try to talk better, but I just, can't. Anyway, a few days ago I had a talk with a complete stranger that lasted more than an hour. It was a wonderful talk, but more importantly, my speech impediment was completely gone, vanished into thin air. I talked **more** than fine, in fact, my words were just flowing out of me, I talked in a very steady tone, like the way I talk to myself in my head. I could express myself and my ideas perfectly! But as soon as I came back home, guess what, plums in my mouth when talking to my parents So I was just wondering, what is up with this, why can't I talk to people I know but perfectly well to strangers?? Shouldn't it be the exact opposite? It's not like "I just don't make an effort to talk well to people I know because I'm lazy" That time I talked to the woman was completely effortless, and no matter how much effort I put before I talk to my parents or peers I. just. cannot. talk. WHYYYYYYYY