Most doctors would prefer to send a letter directly to the SSA, rather than give it to you. Such a letter should not be thought of as a "cover letter." It is a statement of professional opinion, more like a medical consult. Therapists writing will help too. Get a lawyer. The law office will keep track of what records are sent by pdocs and therapists to the SSA.

You want to send your paperwork to the law office and let them forward it to SSA. They can catch anything inappropriate that you might innocently put down.