It really does sound like her mental health isn't great right now. Maybe you'd both benefit from her seeking some form of treatment or taking care of her mental health in other ways. Does she have a support system other than you that she can vent to?
This requires a professional. Find a qualified therapist that does couples, and go. Make it an ultimatum if necessary, couples counselor or you leave. She may need some help on her own, but you cannot necessarily make her seek treatment. You can make both of you go, though.
If she has something like clinical depression, this'll just be a first step towards getting her some help. If she's just taking advantage, this will help you start to sort it out.