Didacticism is a term that refers to a particular philosophy in art and literature that emphasizes the idea that different forms of art and literature ought to convey information and instructions, along with pleasure and entertainment. ... However, some literary texts are entertaining as well as didactic.
The subject is mother and the mood seems very demanding
I would say that the lines which show us how desperate the poet is are the following:
"... helpless in my throes, Biting my truant pen, beating myself for spite--"
As you can see here, the poet says he is helpless, meaning that he cannot find inspiration to write what he wants to write. Therefore he is desperate for some inspiration, which is why he is biting his truant pen in order to show how anxious he is to get his inspiration back.
