how do I stop being insecure within a group of friends?

so uhh i usually feel like my friends are too good for me and that i don't deserve to be with them and idk the reason but i have an idea. i feel like the reason is a bit petty considering it was when i was really young (well, i'm still really young lol) but my friend made me feel like everyone else (in the friend group) was better than me and she made me feel like i was a bad person i guess.

my "friend" fatshamed me a bit and also hinted at how i was weak. she would exclude me from things she'd do with the other friends in the group and when my other friends wanted me to be included so she'd make a "test" for me so that i could join them in whatever they were doing. of course it was about being athletic. I found this thing I really liked and then she took it and said that i could only get it once i did some things for her. it wasn't even hers anyways. and i never even got it since she "gave it to her little sister". she would never tell me her secrets even though i tell her mine. we had a new student in our friend group once and she was a thief. she stole stationary from me but the "friend" took her side for some reason because she liked her? and guess what? she stole a lot of other stuff from other people. they got there stuff back luckily but i didn't get it back because idiot me believed that she really owned those things. i feel like this whole paragraph is petty but eh. i'm really sensitive anyways.

not sure if this is why i'm like this but i usually get upset for petty reasons. how do i fix myself?