I have helped two people file for SSDI and am about to embark on another one. I also have a good friend who is in the process. SSDI is a long and discouraging process. My ex had all kinds of test showing he was disabled, a reputable neurologist etc. But the standard seems to be deny on initial filing. Deny on protest. Often you will get it when you finally get a court date with an ALJ but not always.
What my ex's lawyer and my friend's lawyer said is to file the initial app yourself. Find a reason for the protest, they usually make it easy by neglecting to get medical records from one of your drs., and then get a lawyer for your ALJ filing. This worked well for my ex, did not work yet for my friend and one other friend actually bypassed it all and got it on the first try. His records were no better or worse than the other two so the reviewer must have been having a good day.
My step-daughter has been rejected but because others had a hand in filling out the papers (state agencies that seem to have done nothing) I am going to tackle that now. I know when I first got custody of her when she was a teen there was an SSI app that her mother had filed that apparently was denied. We have no idea if the two state agencies actually filed her adult app or not and they aren't telling either.
It is a difficult process but worth it when you finally get some help. Good Luck.