I am sure that when King Solomon wrote Lamentations 3:22-23 ESV,
The steadfast love of the Lord never ceases;
his mercies never come to an end;
they are new every morning;
great is your faithfulness.
he would not think of several generations later God would say in Jeremiah 16:5 AMP,
For thus says the Lord, “Do not enter a house of mourning, nor go to lament (express grief) or bemoan [the dead], for I have taken My peace away from this people,” says the Lord, “even My lovingkindness and compassion.
Is there a conflict there?
Not when you read it in context. In Jeremiah 15 we see God is speaking about Jerusalem. For King Manasseh was the one that led the people into Baal worship and away from God. He had even set up altars in the Temple of God to worship false gods. Sacrificed his one son by fire to Baal. God is purifying His people. Later on, in chapter 16 you see that once they repent and He will bring them back so that they know His power and might.
So, if it seems like God has removed His mercy from you search your heart as to what you have placed more important than Him and repent. That is what He is seeking.