Song II
Cupid's accounting
The speaker wants all his future happiness paid out now in a single lump sum. He's treating love like a financial transaction—'sterling' means solid currency, and he'll 'absolve' (cancel) future debts.
Full-blown
18th-century term for a flower at peak bloom. Barbauld uses the horticultural metaphor to describe sexual maturity—Florella at her most desirable moment.
The pivot
The poem's 'but' marks the shift from fantasy to reality. Everything before this is conditional ('if'); everything after accepts her rejection.
Constancy bargain
His new prayer: not to win her, but to remain as unchanging in his devotion as she is in her refusal. He's asking to be permanently stuck.
Perverse prayer
The final paradox—he begs Cupid to pity his suffering but never cure it. He wants the wound to stay open.