You are, in general, a serious person, only your seriousness is aimed at what occupies you, and not at what is worthy of attention. You are too busy. This is the whole trouble. Hence your terrible fatigue.
The passage reflects on the nature of a person's seriousness, suggesting that their focus is directed towards trivial matters rather than more meaningful pursuits. This misplaced priority leads to a sense of being overly occupied and distracted by unimportant tasks, indicating a lack of attention towards what truly deserves it.
As a result, this preoccupation contributes to an overwhelming sense of fatigue. The text implies that by redirecting one's seriousness towards more significant aspects of life, one can find relief and clarity from the burdens of a busy, unfulfilling routine.