It’s interesting to hear Jeremiah repent and confess the sin of his people and his nation on their behalf. He uses the “we” in is confession because although he’s held the line, he is still part of the people who have deserted their God and turned to idols. It’s not “their” sin. It’s 
not “their” shame. It’s his as well. In Jeremiah 3:25 he writes, “Let us lie down in our shame, and let our dishonor cover us. For we have sinned against the LORD our God, we and our fathers, from our youth even to this day, and we have not obeyed the voice of the LORD our God.”
Adam and Eve did not obey “the voice of the Lord our God” when they decided to believe and trust the lies of Satan concerning the fruit of the tree of the knowledge of good and evil. In effect all sin is failing to trust God’s words, or failing to obey the “voice of the Lord our God.” Israel had the words of God as well that Moses articulated for them around Sinai. They entered into a covenant with God while on the mountain. But as time went by they exchanged what mattered most for what mattered least. Craigie says, “The nature of the sin, as it is specified at the end of v 25, is the failure to obey God’s voice: that is, the failure to obey the covenant stipulations for the faith of Israel. Obedience to the divine voice should have sprung from love for the God of covenant; when love had been turned aside from God to the deities of the hillside shrines, true obedience was no longer possible, for the promptings of the divine voice were drowned out by the murmurings on the mountains.”[1]
Feinberg says, “How costly their idolatry was in terms of property and even more in terms of the sacrifices of their sons and daughters! And the malady was of long standing. So at the end of the chapter, we see the nation robbed of her vigor, possessions, and dignity, totally cast down by her sin and willing to endure the grief it has caused. As mourners lie down in dust and cover themselves with it, so the nation lies prostrate on the ground (v.25).”[2] I think he’s wrong! This verse is Jeremiah’s repentance on behalf of his people, but they do not respond accordingly. Huey is correct, “If these words had been an expression of sincere repentance and return to God, the rest of Jeremiah’s ministry would have been unnecessary.”[3] Jeremiah prays the penitent’s prayer for his people. He sets an example of prayer for us. Barrett says, “The prayers of Daniel, Ezra and Nehemiah demonstrate the genuine sorrow over national sin and desire for God’s glory that mark corporate confession. These are just a few, and when you include the New Testament examples of Mary, Paul and Christ Himself there is ample precedent to study and to imitate.”[4]
[1] Peter C. Craigie, Jeremiah 1–25, vol. 26, Word Biblical Commentary (Dallas, TX: Word, Incorporated, 1998), 66.
[2] Charles L. Feinberg, “Jeremiah,” in The Expositor’s Bible Commentary: Isaiah, Jeremiah, Lamentations, Ezekiel, ed. Frank E. Gaebelein, vol. 6 (Grand Rapids, MI: Zondervan Publishing House, 1986), 404.
[3] F. B. Huey, Jeremiah, Lamentations, vol. 16, The New American Commentary (Nashville: Broadman & Holman Publishers, 1993), 78–79.
[4] Michael P. V. Barrett, The Beauty of Holiness: A Guide to Biblical Worship (Greenville, South Carolina; Belfast, Northern Ireland: Ambassador International, 2006), 87.