This is not an answer that addresses any game, or game mechanic, because I don't think that is very relevant in this situation. The scope of this question goes far beyond being a problem GM or player, for that matter.
Seek professional help
As someone with a real life degree in Clinical Psychology (2013, BSc), being a part-time teacher that frequently deals with students having ADD and ADHD, and a boyfriend of someone with ADD, I'm not sure if this platform is the right place to find the most comprehensive, or more importantly: the most effective, treatment. If you are serious about taking "ownership of your condition", which is truly great and admirable, I can recommend to seek professional guidance. Talk to your general practitioner, and ask if you can see a therapist that specialises in your condition. I suggest this, because in general people seem to underestimate the difficulties of conditions like ADHD.
There's various ways of dealing with an attention deficit. A qualified therapist is trained to offer (custom) advice and/or guide you in finding your own ways of dealing with your attention span. This way you can find "tricks" that can help you at the table as well. Of course, the actual quality and accessibility of mental health care varies wildly per country. So if that doesn't work out, or doesn't fit your personal needs: some of my close friends have to deal with ADD everyday (similar condition but without the hyperactive tendencies). They found that "alternative" treatments can work really well for maintaining a better concentration, such as mindfulness, meditation and/or yoga.
- Note 1, on first-/second-hand experience: Keep in mind that meditation in itself is mostly just an exercise in observing, while postponing/removing reactions – both emotional and physical. It can, however, have religious intentions or connotations too. If you don't want that, I can recommend Vipassana meditation, or other forms of non-religious meditation. Having read the book by S.N. Goenka on Vipassana meditation myself, doing the occasional meditation, and having a girlfriend with ADD that did the course multiple times (she doesn't take pills), I can say this type of meditation welcomes all nationalities and religions, and more importantly: it enhances concentration (among other things).
- Note 2, on second-hand experience:
Both my brother and a friend – both coincidentally play in a D&D 5e game that I GM – used to have more issues with their attention span as well. After seeing their general practitioner for a while, and some testing, they discovered they have a form of Sleep Apnea. After this diagnosis they took necessary steps to deal with this sleeping disorder, and ever since their attention is much better! This emphasises my point regarding seeking out professional help.
Personal advice regarding the game
In any case, talk to your friends at the table. Explain to them that "simply paying better attention" doesn't work for you, and that you're taking steps to make it work. From a personal standpoint, I would like to finish with: you taking "ownership" requires time and effort. Having a table that understands that, helps a lot. If their attitude is: fix this now or leave, I would conclude these are no friends at all and you find a game with more caring people. Good luck!