i relate to this so much, my a-levels i barely passed in summer and my unofficial gap year has me bored out of my mind between my job and lack of hobbies
my a-level exam revision was done entirely in the days before the real (not even the mock) exams, which was usually incomplete "sessions" going over my notes from class and my teachers' powerpoints. I dont recommend because it was intensely stressful, even afterwards until results day, but i think the reason i didn't completely bomb the exams like my mocks is because of the adrenaline in the exam, my sheet of important stuff to think about/remember that i scoured thoroughly before the exam, quizlet sets, atrocious amounts of coffee
im yapping on a bit now and i seriously do not recommend these methods because it was hell seeing everyone else's perfect revision strategies in the college library while i went to go and waste time doing anything non-productive, feeling guilty nonstop but never just going to fix it. As for the ADHD part, ive been scared of that word for years and i'm terrified and probably procrastinating taking any meaningful step towards diagnosis,, but also at the same time i do personally align with a lot of the symptoms and struggles shared
anyway,, you're not alone in this feeling of not being made for formal education, i hope your resits all go well and i genuinely wish you good luck :-)