OP, I get it. I had a mother like yours. I was also diagnosed with depression, complex PTSD, etc. The only thing that's going to rid you of your ailments is moving away from your mother. While it's hard to do when you're emotionally beaten down, getting a job and a place of your own will be your ticket to better health. I can tell you it will be hard (mentally and emotionally) but it will be worth it. Right now you're using your mother and depression as excuses. Only you can change that. Not an internet forum, not any human resources agency, not law enforcement - only you. The first step is the hardest, go ahead and get it out the way and start those baby steps to true freedom, happiness, and a depression-free life. If you don't, there is no one to blame but yourself at this point.
Now, on to your legal question... the very best thing you can do is never bring your son around your mother. Hopefully your GF agrees. Don't allow your mother to establish any kind of relationship with your son. Aside from that, you'd have to wait until she files something for grandparent visitation rights or does something to warrant an order of protection/restraining order.