From a blog that is reprinting Kafka’s diaries:
Apart from my family situation, I couldn’t live from literature alone because of the slow development of my work and its particular character; in addition, my health and my character prevent me from devoting myself to a life that is uncertain at best. So I have become an office worker at a social insurance institute. Now these two professions could never tolerate one another and accept a shared fortune. The least good fortune in one is a great misfortune in the other. If I have written something good one evening, the next day in the office I am on fire and can’t get anything finished. This back-and-forth is getting steadily worse.
In the office I fulfill my duties outwardly, but not my inner duties, and each unsatisfied inner duty turns into an unhappiness which never stirs out of me.
That last graf would make a good addition to a resignation letter.