I am going through a somewhat similar situation. For us, it is definitely mental illness, likely depression. Although she is being treated it seems like it won.t be enough and she is going to leave. Not because of any problem that she or any of the therapists and docs can identify but just she feels terrible about everything and is looking for an escape.
You mentioned swings of emotion and her feeling depressed. These might be a clue that it does not have much to do with you and that you can’ t impact it much other than to see if she can get help.