It is always interesting to analyse what specific sonnets say about the state of the relationship the speaker is "enjoying" with the object of his affection. Clearly, this sonnet shows that the relationship is not going too well, as the sonnet takes the form of a plea for the mistress to moderate her treatment and criticism of the speaker, as this will have consequences that might be unpleasant for both:
Be wise as thou art cruel; do not press
My tongue-tied patience with too much disdain;
Lest sorrow lend me words and words express
The manner of my pity-wanting pain.
The speaker is clearly suffering great "pity-wanting pain" that describes the difficulty of his situation and how much grief he is experiencing. He advises her to treat him like a "testy sick" man that only receives good news from his doctor, even though he really knows he is dying. He would rather hear lies and illusion than the truth, which would send him into a state of "madness." Therefore, as the poem concludes, it would be better for his mistress to continue acting the same way, even if her "proud heart go wide" in reality. The speaker can only take so much pain at this stage in the relationship.