Jeremiah 17:4
4 And thou, even thyself, shalt discontinue from thine heritage that I gave thee; and I will cause thee to serve thine enemies in the land which thou knowest not: for ye have kindled a fire in mine anger, which shall burn for ever.
Jeremiah 3:12
12 Go and proclaim these words toward the north, and say, Return, thou backsliding Israel, saith the Lord; and I will not cause mine anger to fall upon you: for I am merciful, saith the Lord, and I will not keep anger for ever.
Above are two statements - one indicating that God's anger doesn't remain forever (also see Micah 7:18, Isaiah 57:16, Psalm 103:9), and another indicating that God does have anger that will endure forever.
However, neither of these are absolute statements about the nature of God. Notice they are both conditional, and God is presenting His temporal reactions to certain people based on what they choose.
That is, God makes it clear that you can do things that He will never cease to be angry at, here on this earth. In other cases, that won't be the case. That's not hard to understand.
This does not extend into eternity, because His audience of believers are forgiven eternally speaking (1 Thessalonians 5:9, John 3:36, 3:18, etc.), meaning the "forever" is only in the context of His audience's earthly lives.
There are many such instances in Scripture where "forever" is used to refer to someone's entire life or as long as whatever situation being discussed remains, rather than "forever into eternity" (Leviticus 25:30, 25:46: 1 Samuel 1:22, 2:30, 2:32, 3:14: 1 Kings 12:7: 1 Chronicles 28:9: 2 Chronicles 10:7, Ezra 9:12, Nehemiah 13:1, Proverbs 29:14, Jeremiah 35:6, Amos 1:11, Obadiah 1:10, etc.)
Ignore context, audience, and the rest of the teaching of Scripture, and you can make anything "contradict". This is simply a case of not dealing fairly with the text and practicing bad reading comprehension.