Asking God for purgatory on earth as a grace means that one has to expect suffering. The entire reason for purgatory is to be made perfect for heaven--remember the theme of the over 1,000 entries on perfection-only the perfect see God.
To ask for purgatory on earth allows us three states:
one, purity of heart;
two, pure service to God and His Church without egotism;
three, greater love of God.
May the Lord grant us the grace thus to suffer our purgatory before death rather than after our last sigh. In the evening of life we shall be judged on the purity of our love of God and of souls in God. Garrigou-Lagrange