Why is Hell's sentence eternal?

Why must a God who loves his children send them to Hell for eternity?... if you make a few small mistakes... say smoking pot or sleeping with the neighbours wife you are to burn in Hell for eternity... why?, why can't God forgive some things after a few years or decade or centuries of torture?, why does he turn his back completely to even the smallest of mistakes (ok mistakes that send you to Hell not purgatory) Is God so set in his ways he can't change and forgive once in a while?