Which lines express desperation in this excerpt from sir philip sidney's sonnet 1? but words came halting forth, wanting invention's stay, invention, nature's child, fled step-dame study's blows, and others' feet still seem’d but strangers in my way. thus, great with child to speak, and in my throes, biting my truant pen, beating myself for spite-- "fool," said my muse to me, "look in thy heart and write."

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.

It shows he knows real life experiences and how people live on these harsh conditions