There's a couple of things I want to get out of the way before I actually answer the question.
First, I actually do have ADHD, confirmed via multiple tests, including an EEG. It was never any obstacle for me to get my medical certificate or pilot's license. When my AME asked if I had any preexisting conditions, and I mentioned ADHD, he just waved his hand and didn't even note it down. (Of course, I'm in the US. The medical requirements might be different in other countries.)
Second, the fact that you can spend 8 hours on a flight sim is actually a point in favor of you having ADHD. Sure, "everybody knows" that ADDers can't focus on anything ever, but that's not actually true. When doing some task that actually interests us, we're actually more likely to slip into a state of hyperfocus on that task.
With all that being said, ADHD isn't something you can get, or something you can be cured of. It just is. So, if you can get a doctor to say that you don't have it, that's the same as them saying you've never had it. My advice to you is to ask your AME if it's even a problem in your country, and if so, find a neurological specialist to test you for it.