It is the ghost of Christmas past where Scrooge learns to see the gravity of his past mistakes, to be given a new perspective — seeing past choices through the prism of his current lens. He is given the gift to go into his darkness, to feel an extreme despair and remorse for his actions, and to become aware once and for all of its cause.
But the final ghost, the ghost of Christmas future, is where Scrooge learns his toughest and most intractable lesson. This vision shows Scrooge all the non-blessings his choices have allotted him upon his death. He is not missed nor mourned by anyone; he is only laughed at and mocked.
This vision teaches Scrooge the sum total error of his ways and proselytizes that if he cannot find it within himself to change, he will die a miserable man, alone in his casket, bereft of joy and devoid of peace. He will have left this world a burden, not a value to anyone or anything. His soul will never know its true worth.
It is at this point that Scrooge comes to the gravest of decisions, represented by a singular, overarching resolution — one so undeniable and poignant in its truth that he must accept it once and for all, or be swallowed by his darkness.
The man who can change his mind can change his world.