Jamieson, Fausset and Brown on Jeremiah 15:18

18. (Jr 30:15). "Pain," namely, the perpetual persecution to which he was exposed, and his being left by God without consolation and "alone." Contrast his feeling here with that in Jr 15:16, when he enjoyed the full presence of God, and was inspired by His words. Therefore he utters words of his natural "infirmity" (so David, Psa 77:10) here; as before he spoke under the higher spiritual nature given him.

as a liar, and as--rather, "as a deceiving (river) ... waters that are not sure (lasting)"; opposed to "living (perennial) waters" (Job 6:15). Streams that the thirsty traveller had calculated on being full in winter, but which disappoint him in his sorest need, having run dry in the heat of summer. Jehovah had promised Jeremiah protection from his enemies (Jr 1:18, 19); his infirmity suggests that God had failed to do so.