I don't think the inability to think critically is a symptom of ADHD. Conversely, being prone to addiction to many things that provide instant gratification like, I don't know, gaming, is. If you're saying that you missed the opportunity to learn critical thinking because you weren't able to pay attention in school, I don't think you missed out much based on my experience with school. I don't like to be overly cynical about schools, and I really hate it when I hear people tell others, especially young people, that college is a waste of time, but the single best thing I learned in university was how to do well in school without going to lectures. I don't think the classroom is a good environment for critical thinking because, for me, at least, I need to be put in a somewhat uncomfortable position to force myself to think critically, and you just don't usually encounter situations like that by attending lectures or doing homework. Going to every lecture was an inconvenience to me at the time, mostly because I refused to go to bed before 4am (lol), so I had to explore a creative way to do well in school without going to lectures. I suppose strategy games do put you in situations that are amenable to critical thinking, but there must be many other things in your life currently that can also provide you with the opportunity to practice critical thinking. I believe critical thinking starts with observation. There are probably things in your life you could be doing differently for the better, but you just never considered that possibility because you haven't been observant enough.