Albert Barnes Commentary Psalms 102:9

"For I have eaten ashes like bread, And mingled my drink with weeping," — Psalms 102:9 (ASV)

For I have eaten ashes like bread - I have seated myself in ashes in my grief (Job 42:6; Isaiah 58:5; Isaiah 61:3; Jonah 3:6; Daniel 9:3; Matthew 11:21); and ashes have become, as it were, my food. The ashes in which he sat had been mingled with his food.

And mingled my drink with weeping - Tears have fallen into the cup from which I drank, and have become a part of my drink. The idea is, that he had shed copious tears; and that even when he took his food, there was no respite to his grief.