What simile does the speaker use in lines 11-12 to describe his new state of mind? Does this simile strike you as a good description of joy?

The images brought out in lines 9-12 are the critical points in the sonnet as it changes the focus of the sonnet.  The first portion of the sonnet focuses on a great deal of challenge and misfortune.  Yet, through the invocation of the love subject of the speaker, we begin to see how the power and redemptive spirit of love can be transformative.  The simile which helps to evoke this would be the use of the bird.  When Shakespeare describes "the lark at the break of day arising," one begins to see how there is a certain powerful element within the love of another that can help to reconfigure trying circumstances into ones of triumph and rebirth.  The negative element of thoughts which are described as "despising" is countered with the power of love, which is seen as a type of dawn, "excellent and fair."  The song of the bird, or "lark," is one that recasts negativity and challenge into one that transcends "sullen earth."  It is extremely powerful in its description of joy.