OK I know that whatever you do and no matter how hard you try their will always be some people out there who will not like you even if you've never done anything to them. It really gets me fustrated because its not fair ,for people not to like you for no reason. Some girl at my school doesn't want to stop calling me stupid for no reason its not like I ever called her something, I don't get it. I always try to be nice to her and sometimes she's nice back to me but others she's just so frustrating and I never really done anyting for her to call me stupid , I know that theirs a lot of people out there who act like that but I don't know why and sometimes it makes me think I'm not a good person when I know I am. I hate the feeling when you think someone doesn't like you its so addictive and I can't forget it that easily. Any help or advice??

Don't obsess on it. If you really think about it the people that are shallow enough to not like you really are the ones not worth getting to know. Like people who are racist they are the losers cause they are the ones missing out on things because they limit their opportunities to really learn and grow.
Also she may not really think you are stupid but says that to get your attention
Maybe she is shy and afraid if she calls you a nice name you will know she likes you and that scares some girls to let a guy know they really like them.

First and foremost, its not about you. When (some not all) people are angry about something (parents friends anything) they blame other people for all that's wrongw with them, they want to feel better about themselves by making people around them small. Yoiur probably right there probably isn't a thing wrong with you, its all in her head. So be polite and don't assosiatewith her, if she asked you say "i want be around you till you treat me better." you probably have to tell her that you called me stupid and stuff, and yeah she might say "i never did that"
Trust me I know how this works, I had a friend who's mom was always getting married and her new husband treated her like dirt, but she took all her anger out on me, now I don't know what's going on in her head, but remmber if you did nothing wrong its not about you, its all in her head, its all about her.
Hope this helps.