Why does she keeps saying she wants a divorce? This reflects that there is something in the relationship that is not working for her.

Have you both explored that? From your post, it seems not - i.e. she says she wants to divorce, nothing changes and life continues.. I would not say this is an idle 'threat'.

Take it seriously and explore the 'why' and IF anything can change. Someone doesn't 'threaten' divorce out of nowhere. Something clearly is not working out for her.