I don’t know the biological reason. Some people are sensitive to stimulants. I know I am.
You mentioned you’ve tried everything. I have too but drugs differ when they are combined. I’ve recently discovered propranolol and take it with Adderall and/or other stimulants. It took me a long time to try the combo because I’m somewhat healthy and have great blood pressure. When I did try it, I was amazed at how much it helped me calm my body so my mind can think clearly. I think clonazipam does the same thing to my body but with the risk of tolerance and addiction and a crappy withdrawl. I feel like the propranolol was the missing ingredient with 15 years of failing every ADHD drugs and more. It’s not a benzo but it relaxes my mind and body so I can think clearly on adderall without the peripheral side effects that I can’t tolerate. I psychologically feel better taking propranolol then a benzo too. It’s just a blood pressure pill but it works wonders taking away the anxiety. No side effects either.
It’s worth a shot. Coming from someone who has been on and off meds since preteens years… hope this information helps.