Asking for help with my hurtful younger sister - Assalamu alaikum

Assalamu alaikum. I’m sorry to write such a long message, but I’m really lost and need advice/support from others who understand. My younger sister treats me terribly. No matter how I act toward her or how kind I try to be, she doesn’t respect me. I’ve tried so many things to build a better relationship: I make snacks and meals for her, help with her schoolwork, and even take on some chores that are supposed to be hers so she sees I’m not against her. Still nothing changes. She often says very hurtful things to me or about me. When we’re sitting together she’ll call me fat, stupid (because my grades aren’t as good as hers), or a failure. She used to mock my acne and the fact I don’t have many friends. Once she even accused me of being kafira over trivial stuff. In short, she’s very mean and doesn’t seem to realize how much pain her words cause. A while ago I reached my limit and we ended up in a physical fight. It wasn’t minor - it got intense. I feel so guilty and ashamed. I never wanted things to go that far. Right now all I want is to go check on her and try to fix things. My mother and I have tried different approaches, but nothing has worked. What should I do next? How can I protect my own heart and also try to heal this relationship in an Islamic way? I would appreciate dua, practical advice, or examples from anyone who has been through something similar. Jazakom Allah khair.