The narrator is speaking of the intense desire when love turns into infatuation. It is a dangerous time, when a person can turn against his or her self.
Desire is death, which physic did except.
This is partly what is meant by “Desire is death,” because the speaker is acknowledging the damage of being so far gone in lust. He describes himself as “past cure” and “frantic mad.” These are strong words. He is becoming mad from the tension of love and his inability to control himself.
Past cure I am, now reason is past care,
The references to “physic” and the medical metaphor demonstrate how love can become like a sickness when it is an infatuation. There is no way to get out. This is partly the reason for truth “vainly express’d,” because even acknowledging the infatuation does not rescue one from it.
This sonnet explores the darker side of love, when a person gets in over his or her head. Passion and infatuation can make a person feel out of sync with his or her self, and not in control of his or her faculties. It is scary to lose control, but that is what we do when we fall in love.