These words are an exaggeration that express the great extent of Jeremiah's sorrow. Alternate translation: "is very great" (See: [[rc://en/ta/man/jit/figs-hyperbole]])
Here "heart" represents Jeremiah, emphasizing his feelings and emotions. Alternate translation: "I feel sick deep down inside me" (See: [[rc://en/ta/man/jit/figs-synecdoche]])