I would look into psychiatric assistance dogs quite carefully. I've considered one myself and its a tricky definition. The dog actually has to be trained to perform a task that helps you. For instance, if you forget your medication often, the dog reminds you. If you have a flashback, the dog performs an action that helps ground you in the present, maybe leads you to a safe place, or forces you to sit down until you come back to reality. If you are prone to getting lost due to your disorder, the dog may be trained to help you find your house. etc. Its not enough that merely having a dog comforts your anxiety. Nor, generally, do personal protection dogs qualify. Indeed, I've found several informational sites that state specifically that protection or attack training should NOT ethically be given to PSDs. You can train "bluff" commands- ie tell your dog to protect you, but what the command really means is, "stay close to me, and stay alert". You cannot teach you dog a command that actually means attack though. Once a service dog proves itself a disruption- such as by attacking, or even barking, it is no longer protected by law. A dog that is perceived as dangerous or disruptive can legally be asked to leave and banned from the premises.
It sounds like you are looking for what is termed a "comfort animal" or an "emotional support animal" That is, a dog that, by the nature of its presence, provides comfort to you. It is not trained to perform a specific task that aids your disability, but rather just having the dog helps you remain calm and feel safe. ESA's have some rights- you can keep them in otherwise pet free housing, they may be allowed on public transportation or hotels etc. But they do not have the same rights as service animals- they are not required entrance to stores, restaurants, or public places.
I would make absolutely sure you know which category your dog qualifies for. As someone else with a psychiatric disability, I think the responsible thing to do is to be sure you are completely and legitimately within the law. for what its worth, in the end, I decided on an ESA.
The other thing to do is to make sure you have a doctor who wil back up your need for a service dog. If you ever got into a legal tight spot, you would need a doctor who could say, this is my client, this is her disability, and this is why she needs a service dog.
As far as breeds, from your description, I'm thinking Rottweiler or maybe Bullmastiff. I suppose a Doberman could fit the bill. Some giant breeds come to mind (Danes, Mastiffs) but they are really a bit large for assistance work- particularly as you would want to bring the dog to school and work, where hopefully it could curl up under a desk or out of the way. A bit difficult when you have a 3 ft high, 200 pound dog, you know? A shiloh shepherd might be another possibility. Keep in mind that having a non-traditional breed might mean you have to do more education and fight for access more often than someone with say, a labrador might. Would your anxiety prevent you from doing that? If so it might be easier to go with, say, a black lab, who can still look intimidating, but is also more widely recognized as a service dog.