What are your tips to deal with executive dysfunction when medication doesn't tackle it?

Hello, fellow ADHDers. I'm in the verge of despair and irreversible frustration.

Here is my situation. I'm trying to prepare to pass a competitive service exam in Spain, Europe. I'm from Spain, by the way. So, because it's a competitive service exam, there is always a ratio of 30 or 40 people per job offer. So, if there are 1.000 job offers and 40.000 people to pass that exam, I need to be among those 1000 best marks.

Anyway, because of that, I need to study like 10 hours per day, 6 days per week. I put one day to rest because I don't like to be burnt out. But what happens? It happens that I'm only able to study 2 hours per day, or barely 3 during my best days. And the rest of the day, I procrastinate or do useless stuff, even when I want to study with all my strength.

I tried caffeine two weeks ago, and until today, it worked for 2 or 3 days, allowing me to study during those hours. But other days, maybe I was able to study for 4 hours, or even 5.

So, if you struggled like me and you were able to succeed, how did you do that? How can I force myself to study for 10 hours per day like normal people do with no issues?

PS: I'm not allowed to take any ADHD stimulants. No Concerta, no Elvanse... I only take atomoxetine, since March 2023.