Why is Farrington unable to concentrate on his work in "Counterparts"?

In the story, Farrington is supposed to copy by hand some written materials for the lawyers he works for. It's boring work, so it'd be hard for anybody to focus on it, but Farrington in particular can't seem to concentrate on his work because he's distracted by thoughts of rage (mostly directed at his boss, who just got through chastising him and making him feel worthless) and by cravings for alcohol. Farrington is probably not only dehydrated but also affected by a strong physical need for the drinks that his brain is addicted to.

Even while Farrington is still standing in his boss's office, having been yelled at, this is what he's experiencing:

"A spasm of rage gripped his throat for a few moments and then passed, leaving after it a sharp sensation of thirst. The man recognised the sensation and felt that he must have a good night's drinking."

After this, Farrington sits down at his desk and makes a sincere effort to begin working, but he's overcome again by thirst, and he sneaks out of the office for a drink.

Later, he tries again to focus on his task, but he's embarrassed (having just been chided in front of some clients by the clerk) as well as out of breath from running up the stairs, and ever-so-slightly tipsy from the drink he just consumed. He tries to write again, but his mind is foggy and he gets frustrated with his own errors:

"He longed to execrate aloud, to bring his fist down on something violently. He was so enraged that he wrote Bernard Bernard instead of Bernard Bodley and had to begin again on a clean sheet."

The smell of a client's perfume, as well as thoughts of where he would go drinking later that evening, also cloud Farrington's mind and distract him from the work at hand. His boss then calls him in for another round of castigation, which physically draws Farrington away from his work--and just when he tries to write again, he realizes he'll fail to meet his deadline, and he's overcome by despair:

"He felt savage and thirsty and revengeful, annoyed with himself and with everyone else. Mr. Alleyne would never give him an hour's rest; his life would be a hell to him."