Did He make a mistake by promising to do something He later decided He could not do? No. Did He change His mind? No. Did humans change their ways and change what God said He would do to them? Yes.
Jeremiah 18
6 O house of Israel, cannot I do with you as this potter? saith the Lord. Behold, as the clay is in the potter's hand, so are ye in mine hand, O house of Israel.
7 At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it;
8 If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.
9 And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it;
10 If it do evil in my sight, that it obey not my voice, then I will repent of the good, wherewith I said I would benefit them.
Jeremiah 18
6 O house of Israel, cannot I do with you as this potter? saith the Lord. Behold, as the clay is in the potter's hand, so are ye in mine hand, O house of Israel.
7 At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it;
8 If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.
9 And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it;
10 If it do evil in my sight, that it obey not my voice, then I will repent of the good, wherewith I said I would benefit them.