How do I end this friendship?

I've been friends with a girl for almost three years and throughout it I've always felt like the little person or less important in our friendship. I share so much with her and we have all the same interests and really have become basically the same person in the eyes of our families and friends. However, throughout these three years she's made me feel like absolute trash and I don't know what to do. I just feel like there's been more good times than bad. At the end of last year she decided to drop me and ditch me for another group and this boy she liked, texting me to tell me that she didn't want to be best friends, but took it back the next day. I cried for an entire week because of that. Most of last year I suffered deep sadness and suicidal thoughts because of the way she treated me. The beginning of this year was wonderful and I unintentionally became closer with the group of friends she initially ditched me for than she was. This past four months she told me that she think she has depression and wanted to cut herself. I was of course extremely concernd and tried to convince her to see someone but instead she distanced herself from me and confided in two other girls from the group. That hurt me of course but because she distanced herself I began to hang out with others in the group more. Soon the two girls she confided in came to me to tell me that my best friend said that I wasn't taking her self-diagnosed depression seriously enough and that she didn't trust me anymore. At this point she was being extremely rude and gossiping about others in the group, storming away from everyone at lunch time and saying she was having panic attacks, and generally really making herself look bad. On the last week of school I had a breakdown at school which is extremely unlike me and I left to get some fresh air and my friends later told me that she just sat there and continued to chat away happily while her best friend ran away crying. That afternoon I hit in a side room of our classroom after the teacher put me there and she was just happily chatting to our classmates and telling the teacher I 'went home sick'. I told her that evening over text that I was over it but she wrote a long paragraph saying how she wanted to be there for me but just felt like the group cared more about me than her and was 'selfish' in that moment. I told her I wanted to fix things between us and she responded by ignoring me over our two week holiday and acting like nothing happened on the first day back at school. It's been a week and she's been acting rude, glaring at me when I'm not looking yet acting like we're still best friends. She's also been acting very sad and distant according to a lot of my friends in classes I'm not in.

After that extemely long explanation I've been urged by all my friends to cut her off but I'm conflicted. On one hand, I desperately don't want to be around her and I physically feel sick at the thought of being her friend (no exaggeration) but on the other hand she claims she's depressed although she hasn't seen a doctor for it. Not saying that that's a lie or anything but she hasn't seen anyone and seems to expect me to 'fix her'. She's also got no other friends other than the group that are on my side and I'm afraid that if I cut her off she'll do something to herself or isolate herself. But I'm also afraid if I don't cut her off I'll be even more abused by her and it'll affect my grades even more than it already has. I've had to leave classes she's in because it makes me feel so sick and stressed.

What should I do? (so sorry for the long explanation theres so much more in there too i tried to cut down the past year