Jeremiah 15:18

Why is my pain perpetual
The pain of his mind; his uneasiness for the good of his people, which was likely to last, having no hope of a change for the better: or it may design the pain which they gave him by their reproaches and persecutions of him, which seemed as if they would have no end: and my wound incurable, which refuseth to be healed?
the same thing is meant as before. The allusion is to an old ulcer, or obstinate wound, which no medicine can affect, is desperate and deadly; and such the prophet reckoned his case to be, or however deprecates it, and expostulates with God why it should be so: wilt thou be altogether unto me as a liar, and as waters that fail?
such God cannot be, nor did the prophet think he was; he knew that he was God that could not lie, and that he was faithful to his promises, and would not disappoint the faith, hope, and expectations of his people; but he feared he would be thought to be so by others, by his enemies, who would triumph over him, and say, where is thy God? did he not promise to make thee a defenced city, an iron pillar, and brasen walls? is he as good as his word? is he not like a dry brook, whose waters fail? are not thy hope and expectation in vain, who hast been trusting to him, and depending on him? and it is as if the prophet should say, Lord, let them have no occasion to speak after this manner; nor suffer my faith in thy promises to fail; show thyself to be as thou art, a covenant keeping God, and whose faithfulness never fails: to which an answer is returned in the following verses.