Jamieson, Fausset and Brown on Jeremiah 20:14-18

14-18. The contrast between the spirit of this passage and the preceding thanksgiving is to be explained thus: to show how great was the deliverance (Jr 20:13), he subjoins a picture of what his wounded spirit had been previous to his deliverance; I had said in the time of my imprisonment, "Cursed be the day"; my feeling was that of Job (Job 3:3, 10, 11, whose words Jeremiah therefore copies). Though Jeremiah's zeal had been stirred up, not so much for self as for God's honor trampled on by the rejection of the prophet's words, yet it was intemperate when he made his birth a subject for cursing, which was really a ground for thanksgiving.