It's a cluster of executive-function-related symptoms, any number of which a given person with the disorder can have a little, a lot, or even not at all. (I forget what all of them are offhand, but I have about a half-dozen of them in noticeable amounts, and several others that seriously debilitate many people with ADHD I've never had problems with.)
So while you may be able to "[not] think about it...just keep working hard...and do great things regardless", that's guaranteed not to be true of everyone with ADHD.
The worst advice anyone with ADHD can give someone else with it is "just do exactly what I did, and you'll never have problems again". That goes triple when "what you did" is ignore it.
The best advice I know of is to learn and understand what having ADHD means to you. Then use that knowledge to figure out how best to mitigate the problems it causes. For some, that will be medication. For others (like me), it may mean finding specific interventions for individual symptoms, and learning to live with some of the others.