Regarding hiring an attorney (or a non-attorney SSDI representative-which often times is a former SS employee) they are paid from your backpay. You do not have to have any money to get help. The percentage is very low, and the max they can receive is low (25% or $6000, whichever is lower.)
You may also be able to find a non-profit group that will help for free.
Many attorneys won't accept clients until their claim has been denied in the first few stages. Non-attorney reps will often take clients earlier and provide more hands on help. (With attorneys, much of the work is often done by paralegals.) These fees can be inconsequential if you have a long term disability (and that is generally the case for those who are approved.) If someone will take you as a client early on, and help you get a quick approval, their fees can end up being quite low.
You might want to check into services provided by your local Vocation Rehabilitation Center. They'll provide lots of testing, which will help you document your issues, and even send you to docs, if needed. They might be able to accommodate your disability by providing you with work training in a field that would be something you could manage. You can be entitled to certain benefits as a disabled worker, but maybe not so severely disabled to qualify for SS disability. Could you handle phone work from your own home? They maybe to help you return to work, and if not, you'll have credible documentation.
Have you spoken to an attorney regarding (I'm assuming PTSD) a veteran's claim? I believe some have had success years after discharge. It is not a quick process, though. There's some good info here: