In “Sonnet 147” the speaker realizes he is in over his head in love. He compares love to a disease, a fever that turns him mad and from which he cannot escape.
The beginning of the poem makes the comparison between love and sickness.
My love is as a fever longing still,
For that which longer nurseth the disease;
The longer you are sick, the sicker you become. He describes how he is unable to get out from under the madness of the disease.
Past cure I am, now Reason is past care,
And frantic-mad with evermore unrest;
When love turns to infatuation, you cannot escape. You are lost, because you can no longer reason. He can no longer logically analyze your situation.
At the end of the sonnet, the speaker realizes that his own heart has deceived him.
For I have sworn thee fair, and thought thee bright,
Who art as black as hell, as dark as night.
At first, love seemed like a good thing. It seemed bright, and full of possibility. Yet as time went on, the love became a trap. It closed around him, and he found himself no longer able to get away, and no longer able to control his own actions. This is the danger of deep, passionate love. There is a difference between love and infatuation.
Who is the mistress?