You also need to consider whether the dog has the temperament necessary to be in public places, and whether you have the money to pay a trainer to get the dog up to service dog level. Tasks are actually the easiest thing to train, it is the high level of obedience and public access skills which are the hard part. Public access is the most expensive and time consuming part of training. It is not something you can train on your own, and it needs to be incredibly carefully planned and carried out. You do not train dog for public access by simply dragging it everywhere with you, infact this is the best way to ensure that the dog washes out.
I believe that you would benefit from a PSD, but whether the dog you currently have is suitable is unknown. They need to be trained to such a level that you can stake your life on them. It is not simply a matter of having a well behaved dog. If you are your medical team feel that you would benefit from a PSD, and if you feel that your current dog cannot be trained to the required level, then you would be better off contacting a program, and trying to get a dog from them. This site also has some great articles on how to find good programs and good trainers.