Must be really tough to be in such a complicated and delicate situation.
One thing I would like to say is that it is definitely not your fault. None of us are perfect but you are not responsible for your mother's depression. It sounds like you care a lot about her and her well being and I am sure you are willing to try anything that might help her but the problem is that no one really knows what to do.

Stay strong. I hope everything works out for you.