There are a lot of factors without being a fake SD (which is illegal).
You have to have a condition serious enough that the dog will be actually DOING something to prevent self-harm or harm to others, not just kind of making you feel better by being around. That is between you and your doctor to establish.
THEN -- once it is established you do indeed have PTSD or another condition that can be mitigated by the presence of a dog. . . .
Your dog or another such dog must be of the personality 1000% safe in public, trained in more than just basic manners and skills, the CGC is preferred to be obtained though not required (yet), AND the dog must be specifically trained in tasks to help with your disability.
It is a HUGE liability to take your dog with you. Are you willing to lose everything over taking the dog should it bite or damage something or misbehave and a business owner pursue it? Lots of business owners are fed up with fake or marginally/minimally trained SD's and they are willing to risk a little to stop it.
Are you and your dog willing to deal with what the public will do to you? There are instances of people really being awful to SD's -- touching them, sending their children to touch them, kicking them, knocking the owner dowm and laughing, giving food to the dog, claiming the dog bit them when it didn't, hitting the dog with a shopping cart, etc. Is the dog flexible enough to handle any situation flawlessly and calmly? How about jury duty? Stores with food? Large crowds? Public tranportation? Would your dog EVER bark, lunge, potty, growl, or freak out for ANY reason? It is A LOT to ask of a dog. Most dogs cannot do it. Even ones bred for it and trained from puppyhood by families who are supported by dog trainers and very detailed methods a high percentage wash out of the program.
If your doctor indeed deems a SD as right for your condition, do work with a trainer intensely to evaluate and train your dog. Sure it helps a lot to have a dog with you but it can also be a huge pain and even a risk so you must weigh all aspects.
Cara -- Central CA
+ Shay -- GSD
+Melody Rose & Cubby Shelties