My brother used to stutter a lot and it gradually got better as he got older. He worked with a speech therapist at a young age and all of us siblings were told to wait for him, be patient, and don’t finish his sentences for him. I used to do that with him and also when people made fun of him, it made me more angry than anything in the world. I could only stand up for him when I was around him though, and I didn’t see him much at school since we were 3 years apart. One time, one of my ex-boyfriends came over to my house for dinner and he brought a friend with him. The friend made fun of my little brother at the dinner table and the whole family was just speechless. All of us were so angry, but everyone was too polite to say anything at that moment. Later, we totally trash talked that guy behind his back and I broke up with my boyfriend and I still remember that day when I think of him.
My brother did eventually get better, but his self esteem was pretty low all the time. I hope you can get a therapist or someone to help you so you don’t suffer too much from this.