The term SSD is confusing. It's best to use SSDI or SSI.

You can certainly effectively represent yourself, but it can become very complicated and can be overwhelming. It's not uncommon for vets to receive a 100% VA disability rating and be denied SSDI. There is also the option of choosing a non-attorney representative--they are often former SS employees. They will often help much earlier in the process and since they are only paid a25% of a person's backpay (with a max of 6k), it often doesn't amount to much for them (which is why attorneys generally insist you're far along in the process before taking clients on, so that you've racked up significant backpay.

I represented myself successfully but the process took much longer than if I had used either an attorney or a rep, and I had family support that allowed me that option. It's best to thoroughly educate yourself early on, because once you progress to the ALJ stage, the whole process slows down and your case can be stuck in limbo for months to years.