Hey there,
Im on the spectrum quite a lot too, so I feel your concerns. Im a bit different though, as I mainly have autism (that kind of autism where you sometimes struggle with people, get overstimulated and have obscure interests and think differently than other people). However, my psych aswell forwarded the idea that I could have the inattentive type of ADHD, meaning no hyperactivity, but being easily distractable and so on.
For me, fixed routines help. The better I am living my live and manage my neurospicyness, the better language learning or really living works for me. This means going to bed at latest at 11pm, doing sports and meditation every day, eating healthy, rarely consuming caffeine, no alcohol and other drugs, not a lot of sugar. I’m doing this to the best that I can, but healing is nonlinear, so I sometimes don’t manage - but I try.
If youre not living healthy and putting your body and mind first, you can not learn a language either, even more if you’re being neurodivergent. That is the pill we all - neurospicy or not - have to swallow. Would an athlete who trains for olympia engage in being up until 2am, consuming lots of sugar, stimulants or worse? Be like that athlete. Learning a language is a marathon of training aswell.
Currently, Im also working on getting diagnosed with AD(H)D aswell to get that sweet Ritalin or something like it. Have you ever had an energy drink? I know, it is a bad thing, and Im not doing it regularly, but once had to. It was amazing - all the topics of my studies and other stuff I could remember, no brain fog, full focus and clarity. I managed to write a 4pages long essay for Uni that I had to submit that day in 2,5 hours. It went amazing and I am still proud of it to this day!
My psych told me this is what ADHD meditation could do.
Besides those general life advice, I would advice getting lots and lots of comprehensive input. Its fun and if youre lucky, you might be able to even fully turn your brain off when it comes to watching videos or listening to podcasts. Keep in mind that it should absolutely be comprehensive input for your first few hundred hours, otherwise you will due to the nature of ADHD most likely get frustrated and quit. Afterwards, let the sunken cost fallacy kick in and engage in harder material.
Tl:dr; don’t stress about it too much, put your physical and mental health first (japanese will come way better then), absolutely don’t force the language into you and take it easy and natural. Then, everything will fall into its place.
May peace be upon you.