Analysis of Sonnet 147: My love is as a fever, longing still

My love is as a fever, longing still
For that which longer nurseth the disease,
Feeding on that which doth preserve the ill,
Th' uncertain sickly appetite to please.
My reason, the physician to my love,
Angry that his prescriptions are not kept,
Hath left me, and I desperate now approve
Desire is death, which physic did except.
Past cure I am, now reason is past care,
And frantic-mad with evermore unrest;
My thoughts and my discourse as mad men's are,
At random from the truth vainly expressed.
    For I have sworn thee fair, and thought thee bright,
    Who art as black as hell, as dark as night.