Friend Help?

my friend and i were supposed to do this thing, and we have prepared it for months. And now she tells me she's doing it with someone who asked her 4 days ago.. and it just kills me because we were supposed to do it together, and she is one of my best friends and i am now sooo mad at her!! and i feel like i will be left friendless wihout her. And i didn't do anything wrong, she is the one who screwed up