IS PROPHECY GUARANTEED TO COME TRUE?

Jeremiah 18:7-10  At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it; 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.  And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it; 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. (KJV) II Peter 1:19  We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts: (KJV)

The Jeremiah has nothing to do with prophecy.  Jeremiah is saying that if God tells a nation that their sins will bring judgment, and that nation repents of its sins, the judgment will be lifted, and that if God promises a good nation a reward, and they turn to evil, the blessings will not come.  One may ask why God apparently can't tell if the nation will turn to or from evil.  God does know if a nation will do so, and the threats of punishment and promises of reward (and their lifting) are tools used to prod these nations in the right directions.  Not a contradiction.