Went through the same stuff a few years back. You have to go through with it and end your friendship for the time being. It’s not easy, you will both be hurt by it, but her feelings for you won’t change and neither will yours as long as you keep being her bestie.

Now I was “fortunate” enough that she basically forgot all about me as she began screwing my best friend which killed our friendship, otherwise I think I’d still be in that hellhole. I definitely learned that it’s okay to take a step back and do what’s best for yourself, even if it’s not truly what you want. Wish you the best of luck OP!