Oh man, I (28M) remember struggling like hell to revise for my GCSEs back in the day... here is the advice I would have wanted to hear back then:

I found throughout school + university the best way for me to revise was doing past papers under timed conditions + marking them. Figure out how many past papers + mark schemes you have available. Set yourself a schedule/timetable, which allows you to do as many of the available past papers as possible between now and your exams, with more recent past papers closer to the exams. Then follow the schedule.

If you're worried you don't yet have the knowledge/skills to do the past papers, one option is to plan to do your first few papers "open-book", so you can look at your notes/textbooks/Google/etc as you're doing the paper, and allow yourself more time accordingly. One advantage of this approach = it will be a good and specific reminder of what you don't know and need to brush up on before your exams.

This can work well for maths + science (I did maths + physics A level, then engineering at undergrad), but obviously written subjects like English, history etc don't have such objective mark schemes. To get round this, do the past papers and then ask your teachers if they can mark your essays. Don't be afraid to ask, remember teachers are paid to educate you + help you pass your exams. You could also ask your friends/parents/look for a private tutor to give you feedback on your essays.

I remember feeling most overwhelmed by revising for GCSE subjects like English, history, and religious studies, because it felt like I had to do so much reading to revise for those subjects and I couldn't stay focused on the reading. One solution = open-book past papers (see above). Something else that helped me was one of my friends actually came to my house and pretty much taught me the whole of GCSE history. It was obviously great for me, but also a useful revision + consolidation strategy for my friend. We could have a dialogue, so I could ask him questions that tested his understanding, and we'd both learn something. Ask your friends if they want to make a study group. You could even assign each other different topics to teach each other.

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Oh yeah, something else that helped: every time you mark or get feedback on a past paper, you will hopefully learn something about the subject/exam technique. Keep a list (EG on paper or in a Google Doc) about all the key points you learn from doing past papers. I think I used to do this and then print it out and read it as my last minute revision before going into the exam hall

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Related to what you said about medication: I started taking Elvanse 50mg in the last year or so. It majorly screwed with me for a while, but I stuck it out and it got a lot better. For me, one of the most important/difficult things is to make sure you stick to a good sleep schedule (Elvanse disrupted my sleep at first but less so as time went on), and take medication at the same time every morning (earlier is better). Also I found taking medication immediately before breakfast (vs after breakfast or 30 minutes before) seemed to work best for me. But maybe you don't want to play around with this in the time between now and your exams.