It's anxiety, pure and simple.
Yes, you did a shitload of drugs and you basically followed no HR principles. I still don't think that's the entire problem here.
I've abused the fuck out of my mind and body during the last four years and I feel I have some experience in this area. Far worse than the after-effects of any drug are the effects of: anxiety, neurotic fixation on perceived deficits, depression, and sleep deprivation. The brain is far, far, far more plastic and resilient than you think.
My recommendation is therapy and counseling FOR ANXIETY. Stop doing *ANY* other drugs. This means no weed, no psychedelics, no Xanax, and no Adderall if you can help it. Start working out as well. You won't want to and you'll hate it, but do it. You can take the SSRI if you want, but either take it and stick with it or don't do it at all. Hopping on and off of an SSRI is no good for anyone.
You need to start reading about different anxiety management techniques and find some that work for you. A lot of PTSD therapy involves coping with intrusive / invasive thoughts and those strategies can help you with the fixation on your perceived deficits, even if you aren't suffering from PTSD.
I have a good friend who is convinced that his brain was damaged because one time he took two Etizolam at once. There's no chance he's suffering a cognitive defect from that, but because he's fixated on this idea that he's not as sharp as he once was he's finding "evidence" of his brain damage all over the place. It's pure confirmation bias, plain and simple, and I guarantee that's what you're dealing with as well.