The sudden blow that comes from within in “The Crack-Up” refers to the realization of a gradual process of breaking down that takes place inside the individual.
External blows, those blows that come, or seem to come from outside, don't show their effects all at once. Internal blows, the ones that come from within, the products of the mind, are not felt until it's too late to do anything about them. When one eventually feels their effects, they induce a sense of finality, a sense that one will never be as good a man again.
The first kind of blow happens very quickly, whereas the other kind happens without your knowing it. But it is nonetheless realized suddenly. In other words, when you finally become aware that you're breaking down from within, it hits you hard in the face. This is the kind of crack-up that Fitzgerald has experienced in his own life.
Over time, he's come to feel increasingly isolated from those around him, from the things he used to enjoy. Whereas he was once quite gregarious, he studiously avoids other people whenever he can help it. Now that he's alone, Fitzgerald has become insulated from the ordinary cares of this world.
Although Fitzgerald speaks candidly of the realization of his cracking-up as being “not an unhappy time,” he readily confesses that the two years in which he was in this state involved a lack of direction and a loss of confidence in his independence.
In addition, he started hating people for no good reason and becoming bitter about trivial things such as the sound of the radio or advertisements in magazines. And for good measure, he found it difficult to sleep and hated the day because it went towards night.
Whatever Fitzgerald may say otherwise, it seems fairly obvious that his internal breakdown, or crack-up, was anything but a happy experience for him.