I am BP and suffer from dementia and chronic pain from major car accident 5 yrs ago. I was diagnosed Bp in June 2005 applied for ssdi oct 2005 and was approved by Feb 1 2006 and got back pay as well. I did not have a lawyer and everyone asked how I did it. My answer
Take every Dr's note ever written. EVERY piece of evidence you have. Every report from your job in your decline in salary or productivity. YOU HAVE to PROVE that you can not function in ANY job you have done in the past YOU HAVE TO PROVE you can do it now nor can in the future. I had as BOOK
6 inches thick of every medical record possible. Letters from my DR's, employers, friend and family members.
It worked for me and good luck to you.