"Once again a long time has gone by in which I have not been able to concentrate on the slightest thing -- I will now try to get started again"

"Eternal child"

It strikes me that Kierkegaard had ADD as discussed here goo.gl/otcRc6

A little tongue-in cheek, but could Existentialism ≈ ADD

Existential angst may not entirely derive from "a negative feeling arising from the experience of human freedom and responsibility" but could be the call of the real world to a distracted soul.

Can explain a few other things:

Camus' car crash

Sartre sending his Nobel rejection letter too late for the decision

Litres of coffee consumed and cartons of Gauloise smoked