As God so often does, He attaches encouragement to His rebukes. God is in fact reminding the prophet of his original commission, as recorded in Jeremiah 1:18-19:
Today I have made you an armed city, an iron pillar, and a bronze wall against the entire land—the kings of Judah, its princes, its priests, and all its people. They will attack you, but they won't defeat you, because I am with you and will rescue you. (Common English Bible)
It was the worst of times back then, yet not so bad as the coming Tribulation that we face today. God demands that His people be faithful. Should we come to doubt God's reliability and His faithfulness—should we come to feel He has abandoned us, His people—we may want to think hard about God's words to Jeremiah: “If you change your heart and return to me, I will take you back. Then you may serve me” (Jeremiah 15:18, New Century Version).

The ever-faithful God will not forget His plans for us, even though we may come to forget His promises to us.