Justice? Or Just a Myth?

In Plato’s The Republic, Socrates makes the point that when the parts of the soul are in balance there is justice and that imbalances lead to injustices. But even if true, what does that say of justice itself? How do we achieve a just world, a just society, or even a moment of justice? Especially when every indication is that we are living in an unjust world. Why do we commit to justice, then let injustice fester and grow?