As someone else mentioned, mental health is a continuum and disorders are constellations of normal traits that have become exaggerated to the point that they interfere with one's life.
This is the main reason why self-diagnosis is discouraged, because everyone has traits that resemble disorder symptoms.
For example
Hypochondriasis isn't simply thinking that you are ill in some way, it's interpreting minor symptoms as a serious illness. It isn't simply anxiety of about being sick, it's impending doom anxiety.
Simply self diagnosing doesn't make you a hypochondriac. Getting a headache and going to an oncologist to get an MRI because you know you have brain cancer makes you a hypochondriac.
Not "ha-ha, maybe it's a tumor", but "I'm getting a second opinion from someone who takes me seriously".
ADD is also an interesting case since difficulty concentrating is actually a feature of a lot of disorders. Anxious people, depressed people, manic people, and psychotic people all have difficulty concentrating. Concentration is a frontal lobe function and the frontal lobes are the most sensitive part of the brain and the first to start malfunctioning when something goes amiss
If I remember correctly, what separates ADD is that stimulation helps a person with ADD concentrate, while for other people it distracts them.
Sparkimus Prime said:
Some of us, like me, are also agoraphobic, which is actually the fear of being caught in a situation you can't escape from rather than fear of "wide, open spaces", though that is obviously a subset.
As I recall, it's specifically the fear of places where escaping would be difficult or embarrassing... in the event of having a panic attack.
That's why Panic Disorder (where you essentially have random Panic Attacks) frequently leads to developing Agoraphobia. You can't predict when you'll have a Panic Attack so the only amount of control you have is avoiding places where it'd really suck to have one.