Two things occur to me: One is that she doesn’t want to divorce you, but maybe she wants you to divorce her! Is it possible that she can’t manage to leave but is (perhaps unconsciously) setting you up to do the leaving? She must know that the relationship can’t deepen and strengthen if she isn’t willing to talk about the issues that are troubling to her. It may also be that she would rather see herself as a “victim” (a person who has been left), rather than see herself as a “victimizer” (a person who leaves and hurts someone who loves them). She may not understand that she doesn’t need to take either role. Reasonable people can decide their relationship isn’t sustainable and can separate amicably.
The other possibility is that she has an undiagnosed and untreated mental illness. The fact that her moods and feelings change so radically points to that possibility. If that’s the case, an assessment by a qualified mental health professional would help determine what is wrong and how best to address it.
Both possibilities require her cooperation to fix. The relationship as it is cannot last. If she can’t bring herself to leave by divorce (instead of periodically disappearing) it is important that she be honest about that so the two of you can separate without adding more pain. If she can’t be, then you are left to make the decision.
If there is a possibility of illness, she needs to be willing to get that assessment and to perhaps follow recommendations to make it possible for her to be in a loving relationship with someone who loves her.
In either case, your challenge is to ask yourself how much longer you are willing to be buffeted back and forth by her behavior. Unless she is willing to cooperate in changing the terms of your relationship, you are always going to be walking on eggshells so she won’t “leave” again. That’s got to be very hard on you.
I wish you well.