Sorry you're having such a hard time, especially when it sounds like you're doing your best to get help. I wonder if you would consider ringing the BeyondBlue help line (at the top right of the screen) to see what help they can recommend for you? Furthermore, if you're comfortable doing so, maybe another visit to your GP to let them know that your mental state is declining. They may be able to refer you to other relevant help in your area.
What do you think might improve things with your MIL? Is this something you feel able to talk to your partner about? If you have a good relationship I'm sure he doesn't consider you to be a burden. In a good relationship you lean on each other when times get tough.
Kind thoughts, Katy