It is of the LORD's mercies that we are not consumed, because his compassions fail not. They are new every morning: great is thy faithfulness.
Exodus 34:6-7
And the LORD passed by before him, and proclaimed, The LORD, The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth, Keeping mercy for thousands, forgiving iniquity and transgression and sin,
In the depths of the covenant curse, the poet turns to the Exodus 34 divine character declaration as the ground of survival. The Sinai self-disclosure established mercies, compassions, and faithfulness as the LORD's constitutional character — the same attributes Lamentations 3 invokes as the reason the covenant people have not been utterly consumed. The 'new every morning' mercies are the daily installment of the Exodus 34 'keeping mercy for thousands': the covenant faithfulness proclaimed at Sinai is the inexhaustible source from which the lament draws its hope amid judgment.