Separation from God is the chief punishment of Purgatory. Wherein Purgatory differs from Hell.
All the pains of Purgatory arise from original or actual sin. God created the soul pure, simple and clean of all stain of sin, with a certain beatific instinct towards Himself whence original sin, which the soul finds in itself, draws it away, and when actual is added to original sin the soul is drawn yet further away. The further it departs from its beatific instinct, the more malignant it becomes because it corresponds less to God.
There can be no good save by participation in God, who meets the needs of irrational creatures as He wills and has ordained, never failing them, and answers to a rational soul in the measure in which He finds it cleansed of sin's hindrance. When therefore a soul has come near to the pure and clear state in which it was created, its beatific instinct discovers itself and grows unceasingly, so impetuously and with such fierce charity (drawing it to its last end) that any hindrance seems to this soul a thing past bearing. The more it sees, the more extreme is its pain.
Because the souls in Purgatory are without the guilt of sin, there is no hindrance between them and God except their pain, which holds them back so that they cannot reach perfection. Clearly they see the grievousness of every least hindrance in their way, and see too that their instinct is hindered by a necessity of justice: thence is born a raging fire, like that of Hell save that guilt is lacking to it. Guilt it is which makes the will of the damned in Hell malignant, on whom God does not bestow His goodness and who remain therefore in desperate ill will, opposed to the will of God.