We as humans tend to believe everything that we are told. Especially, when those words are negative. Such as ADHD, Ugly, Dumb, stupid, Retarded.
Some questions become mental doubts in all that you do such as "There is something wrong with you, Your clumsy, Your slow, You always make mistakes".
Then we turn the negativity into perpetual self replicating doubts. Such as I'm bad at math, I'm not good in school, I'm not a musician, I'm not athletic, I'll never do anything good, and etc. These self doubts often times have their origin from an outside source. Such as a medical doctor proclaiming a lie such as ADHD or OCD. Even if that doctor believes his/her diagnosis is true. The child will now forever believe that there is something wrong with them. When the truth is there is something right with them.
The cool people proclaim you to be anti-cool and never shall you be. The beautiful people judge you to be less than handsome and you choose to believe you are less than pretty. All because of a false proclamation. The same with intelligence and being athletic.
Know yourself is by learning about yourself. Finding your strengths and focus upon them. Discover your weaknesses and grow them. Use your intention and attention and focus your energies into becoming a positive force for yourself. Then and only then can you become a positive force for the benefit of the world.