Is there a reason you didn't apply as a child on one of your parent's work records? Please research to see if you can still qualify for SSD on one of their work records since you have both severe physical and mental disabilities that began prior to your 22 bday. It wouldn't hurt to seek the help of a disability advocate and/or attorney as well...

http://www.ssa.gov/pubs/10026.html#older-children

It can be harder to qualify the younger you are, but your docs seems supportive and the combo of disabilities will be hard for them to ignore. Some states having matching funds for SSI recipients, as well. If you haven't applied for Medicaid and food stamps yet, please check into those benefits as well.