The speaker begs the beloved to abandon him now, while sorrow is already unbearable, rather than inflict a secondary cruelty that prolongs the pain.
Sonnet 90 reframes abandonment as an act of mercy. The speaker is already drowning in misfortune; the beloved's departure would be kindest if delivered swiftly alongside other griefs, creating one catastrophic blow rather than prolonged agony. The speaker begs for immediate execution: strike him now while he's already broken, not later when he's healed. The beloved becomes an instrument of mercy, and departure an act of compassion. Cruelty and kindness collapse into each other.
The couplet contains a dark paradox: only the beloved's loss is truly unbearable; all other sorrows are trivial by comparison. This establishes the beloved as the sole source of real pain. The speaker achieves a kind of stoic peace—if the beloved leaves, it doesn't matter what other griefs follow, because none of them matter. The beloved's departure is both the worst possible and the only thing worth mourning absolutely.
You're already drowning in failures and rejection from others. You ask your partner not to wait until you've finally recovered and regrouped—leave now, while everything is already ruined, so you only have to break once instead of twice.