What is the purpose of the metaphor in these lines from the passage? Of those fierce darts despair at me doth throw: O make in me those civil wars to cease; I will good tribute pay, if thou do so.

The writer is talking about how despair and sadness hurts him. He's saying that sadness wounds him as fiercely as darts. He desperately wants this emotional torment to end and says he would almost do anything in order to do so.