I really hope you get your SSDI, but in case you don't:
The appeal, after the Administrative law judge (ALJ) hearing denial, has 3 possible outcomes:
1. The appeals council agrees with the ALJ and agrees to also deny your claim
2. The appeal council feels that you were denied unfairly or evidence was not properly considered. Then, they will send it back to the judge (a remand), with instructions (like x evidence needs to be weighed higher in decision) and you will have another hearing. The judge, with new instructions/considerations will make a decision. You may win at this point, or get denied again.
3. The appeals council feels the judge was incorrect in their decision and they grant you a winning case (this is rare)
*If denied at this appeal, there are 2 more, higher appeals you can go through. If those are denied, you have to start over from the beginning. Any "back pay" you were accruing while waiting, gets wiped out. Start from scratch.
start a new claim ( you can have two running concurrently)
My understanding, is that now (like, in the last year or 2), you CAN have 2 cases running at once, but can only start the second one AFTER a decision has been made from the appeals council stage (if I am incorrect, someone please let me know!)
It was, prior to this, that you could start a new claim (alongside current claim) as soon as you get denied by the ALJ
So, you can keep appealing up to the highest level. If you win, you get back pay as far back as your "disability onset date" (which is up to 12 months before you applied, it will be on your paperwork, the exact date)
You can start over, but you will lose all back pay
Or, you can follow all the appeals with first claim, AND file a second, new claim. You could potentially win one, both or none
(I hope that all makes sense!

)
ETA: If you get denied, you would need to find out if your current lawyer even does the higher appeals, or if u need a new lawyer for this. Also, find out if they think it would be worthwhile to keep appealing, or just start over. I would suggest, if you have other diagnoses, physical or mental, do all you can to strengthen up those on paper, along with ME/CFS, if/when you decide to re-apply.
ME/CFS is very difficult, as a stand alone, to get approved for SSI or SSDI.
If you have not researched already, this may be helpful, as to how social security defines disability
https://www.ssa.gov/disability/professionals/bluebook/AdultListings.htm
https://www.ssa.gov/OP_Home/rulings/di/01/SSR2014-01-di-01.html