Thinking good thoughts for you.
I feel like long-term friendships can become really toxic, but are the hardest to let go. I mean, you start being friends with someone when you're 15 years old (making that number up, this is all the general you), and it doesn't matter that neither of you is fully aware of all the societal niceties and that maybe one or both of you can be a little selfish sometimes. I mean, you're both 15! Teenagers are notoriously clueless and can be really selfish at times. The only thing that matters is that you're super close, and you're building up a shared history.
Fast forward a decade or two, and you've gradually diverged. One of you grows up, learns how to be a fully functional adult, starts thinking about how their actions will affect other people. The other one still acts the way they did when they were 15. But you've been friends forever, and you have all these in-jokes, and stories. At what point do you say, "wow, my friend hasn't had much personal growth since we were 15."
If you'd met a friend like that as an adult, you very likely wouldn't have a close friendship with them. But, because when you met when you were both clueless teens, that friendship is already firmly in place. It's like the frog sitting in a pot of water on the stove. You don't realize it's happening, because it's so gradual. Except one day you're submerged in boiling water.
I don't necessarily know that this friend is truly toxic (although she certainly is acting toxic in this situation). But I can certainly understand how it might be hard to give up on a close friendship that has lasted since you were both a lot younger.
Hugs, nemesis. I think that you're handling this with great aplomb. Who knows? Maybe this is the wakeup call that your friend needs. If it isn't, you might think about phasing her out of her life. Life's too short to stay friends with selfish people.
My best friend and I have been friends for almost 20 years. We're still friends because neither of us would pull this type of #$! We have other friends from HS that we've let slowly fade from our lives. Because we grew up, and they didn't.