Constantly avoiding parties?

So im a high school guy and i have friends, i am very social at school but whenever my friends plan to hang out after school, i never go, i just tell them im busy even though i never am. I just have sort of a fear i guess. But i feel like such an as*hole a lot of times when i opt out. Like one time recently one of my friends got injured and was out of school and i hadnt seen him in like a month and we planned to hang out at his house and see him, but i didnt go. He probably kinda lowkey missed me, yet i just couldnt bring myself to go. Im kinda scared to tell my mom whenever we are planning to hang out because ive never actually done it before and she will be kinda like wtf, also she has to take me because i dont have a car yet. I feel like i have some degree of social anxiety, not much, only in certain situations. And im not one of those antisocial nerds who are deathly afraid of socializing and sit on a computer all day. Im cool i swear. But why do i always not go to parties or socialize after school? It pisses me off when i dont go because i feel like a jerk, but at the same time i dont really want to go?