For I have eaten ashes like bread, and mingled my drink with weeping,
For I have eaten ashes like bread. The "For" introduces the ground upon which his enemies reproach him (Psalms 102:8) - namely, his great misery, notwithstanding his piety. The enemies sneer at religion itself in the person of its suffering representative. "Ashes" represent mourning; they were cast upon the head in sorrow, while the mourner lay upon them: not that he literally ate them (2 Samuel 13:19; Job 2:8). Jerusalem is thus represented sitting upon the ground (Isaiah 3:26; Psalms 44:25; Jeremiah 6:26; Lamentations 3:16). The phrase is poetical, like Psalms 42:3 (cf. Psalms 80:5). So the parallel clause.
And mingled my drink with weeping - I do not intermit shedding tears even while I eat. As drinkers mingle water with their wine, so my tears fall into the cup out of which I drink.