Friends are destroying my mental health

I’m about a month into my senior year, and I honestly don’t know what to do anymore. My friend group has been making me feel excluded for a while now, and it’s gotten significantly worse this year. I’ll find out about plans after they’ve already been made, hear about things everyone else was included in, or feel like I’m only being invited as an afterthought. I’ve tried making more of an effort and talking about how I’ve been feeling, but I still feel like nothing has really changed.
I recently ended a friendship with someone I’ve known for years because I felt like she repeatedly dismissed my feelings and made excuses when I tried to explain that I was genuinely struggling. Since then, it feels like she’s gotten other people against me, and I’ve become even more isolated from the rest of the group. I know I haven’t handled everything perfectly, but I feel like I’m being blamed for reacting to something that has been hurting me for months.

It’s gotten to the point where I’m genuinely scared to walk into school because I’m afraid I’ll have no one to talk to or sit with. I feel incredibly lonely, and this is seriously affecting my mental health. I’m now considering transferring schools even though I’m already a month into senior year. I would be starting over and wouldn’t know anyone, which is scary, but staying where I am also feels really difficult.
I honestly don’t know what to do. I’d really appreciate advice from anyone who has dealt with a similar situation or transferred during senior year.