:/ GOOD GRIEF. Sorry to hear this happened to you, truly.
I'm going to try and spin it for you though. Yes, it sucks that your MOH couldn't step up to the plate and keep her promise to be your MOH, which by the way, as we all know is an absolute HONOUR. That being said, rejoice that she
told you with 360 days left before your wedding. That's a lot of time to figure out the MOH situation, if anything needs to be figured out. It's better for her to step down now as opposed to a) leaving it to very last minute to step down or
b) staying and doing a slack-off job as your MOH, which is worse than having her step down.
Consider this a blessing. Hopefully, your friendship with her will triumph somewhere down the line, but for your wedding, you need to be surrounded by people who truly love and support you. ;) Just my humble opinion...