I would contact an attorney who specializes in SSDI. Trust me, it's more than worth the percentage they get from your first check. When I filed I was too sick to handle phone calls, correspondence, etc. The attorney contacted my physicians directly and submitted all the necessary information which is very helpful because they are experienced in knowing exactly what SSDI is looking for. I was fortunate enough to be approved within months of filing.
If you are filing for long-term disability from your company I would also suggest contacting an attorney. Again, paying a price to have an attorney review all your forms and information before it's submitted is priceless and much less than paying them to prepare an appeal for a denied claim. I was initially diagnosed with CFIDS and used an attorney who has experience filing claims for this condition.
Here's a forum that offers great advice and lots of information on LTD: http://www.disabilityinsuranceforums.com/categories.cfm?catid=4
I totally understand how stressful and overwhelming all of this can be, especially when you're probably even too sick to deal with filing a claim.