Buddy if you are waiting for a Sign, this is it. It'll never get more concrete than this message I'm typing for you right now. Having a lot of doubts is common. It wasn't truly real for me until I started medication.
My broad advice is to find a good psychiatrist (and don't be afraid to switch if you aren't happy) and dig as deep as possible for evidence both for and against. Go in with confidence that you have ADHD symptoms, but keep an open mind since there are alternative explanations. A diagnosis of "no you don't have ADHD it's actually ____" is also important information to know, and you will regret letting it drag out if you do have ADHD.