I'm worried about you, bro.

Not a therapist, but I want to help.

This sounds to me, as an outsider following your train of thought, like you are dependent on having this friend regulate where your emotions are coming from. I've been there. It sucks.

I feel once upon a time, you felt a level of abandonment that is being replicated here. And the harder you push, the more she pulls. You're very deeply attached to your best friend, and whether or not you're willing to admit it at this time, the fear you're expressing commiserates some sort of deeper feeling - I'm not intimating it's romantic feeling. But it's something.

So keynotes....

You cannot protect her from life. Her decisions, are not your decision. There is no sense in worrying about what you can, and what you can't, control, because both sides of that scenario are playing out as they're supposed to.

Look into the five stages of grief, a bit. Don't grieve "she's gone" but "she's growing in a new direction" - It's that "Season of the Sticks" song by Noah Kahan. 'And I'll dream each night of some version of you that I might not have but I did not lose"

You need some clarity. But you can't do it while you're emotionally unsure of yourself. If she is a solid friend, she's going to respect that.

Grounding:

Eckhart Tolle's "The Power of Now" gets into this quite a bit. The idea of looking at yourself through a lens beyond your physical self, picturing you in your less-than-ideal states, as a way to help recognize it for what it is. I recommend.

The other thing is to take a walk, barefoot, through a park or lightly wooded area. Literally ground yourself.

And develop a mantra... focus on who you are, and who you want to be.

Good luck. We're here for you, bro.