My best friend has corazón cancer and is having surgery. She dicho she probably won't live. What do I do now?

She made me promise not to do anything stupid if she didn't make it, but I don't think I'll be able to do that. She's my best friend and I might lose her. I've cried a ton already, and she is having the surgery tomorrow. I have no idea what to do! I've tried to stay optimistic, telling myself of course she'll live and she'll be perfectly fine...but it isn't working. How do I cope?