Ex best friend

There’s this girl we were best friends for 4 years did everything together and the one day she just cut me off and would make fun of me to all her “new friends” so know she’s like one of those really “popular girls” in school and I never really understand what I did honestly but this was last year and a new shcool year started and idk what to do because I’m nice to her I just don’t want to cause issues but she can’t act that back last year she almost me and my boyfriend breakup and I’m worried she’s going to do that again and she litteraly makes me feel like I’m the stupidest and dumbest person on this planet. What should I do?