Just another note to let you know you've been heard.
I've been in a similar place--I grew up without a lot of friends and when I was in the 5th grade over heard one of the girls that I thought was my friend say "Oh, no here comes lizthefair*" when I was walking up.
It's possible she meant nothing by it--or she was trying to make friends at my expense or a hundred other things--I'm sure she has no memory of the event.
What sucks is, I spent years subconsciously believing that all my friends were only pretending to like me. This made it hard to build deep relationships
The thing that helped me was uncovering where the fear came from and naming it. Now when I feel a bit insecure in a group I remind myself that I'm not in the 5th grade anymore. In groups of good friends I tell the story because in the light of day it's just a funny "kids are mean" story. It loses it's power.
Maybe you have a similar story in your past?
*obviously, she didn't really call me lizthefair