Ouch. Sorry this is happening.
Is it possible to give her the space she needs without having her leave?
Is she feeling like she's suffocating, so she feels she needs to go? Can you let her be alone, or is that not possible for you?
Or does she just want out? Sometimes, and I'm saying this from personal experience, "I need to be alone" is the strangled cry of a person who's really unhappy in her relationship and doesn't know how else to frame it. That was me a few years ago (only the tables were turned -- HE was the alcoholic)
Can you two try counseling to see what's at the root of all this, and see if there's ANY common ground where maybe you could both give a little and not separate except as a last resort? Have you ever been to an Al-Anon meeting, where the families of alcoholics talk about things just like this? It can be really life-saving. There's a lot of good ideas in those rooms, and a lot of peace of mind can be gained.
Good luck
GL