One of the most insidious tricks mental illness plays on a person is to convince them they're not really ill, and this is only bolstered by internalised misconceptions of how mental illness should look and feel.
Similarly to the author, I assumed for years that it was normal to feel a constant sense of worry and low level dread, or to fixate upon imagining the worst possible scenario if a friend or loved one couldn't be contacted.
Anxiety doesn't always mean a person appears shy and withdrawn, or suffers from panic attacks. It can manifest in a variety of different ways, all just as real and debilitating for the individual sufferer.
Imagine that sense of imminent danger you get when your body has a “fight or flight” moment. Now, imagine carrying that surge of panic and adrenaline with you constantly.
http://thetab.com/uk/bristol/2016/05/09/ugly-misconceptions-anxiety-25716