The way I see it is this: All is Grace, we must remember. The only reason that any of us, at all, can repent and turn to God is because of His Grace.
At death, God takes His Grace away, fully, from those who reject it on Earth.
This means that they are working with only their weak, fragile sense of human will to choose right and wrong. Now, for the sake of argument, let’s bypass the freewill conundrum by going ahead and assuming that, all things being equal, a human being could choose God even without God’s Grace drawing them. We must keep in mind that humans who are imperfect upon death ALL, both those who die in Grace and in Mortal Sin, undergo pain so intense (whether in Purgatory or Hell) that none of us here on Earth can even begin to imagine either pain. They will also have the knowledge that God Himself has permitted this fate to befall them.
Now, without God’s Grace, I think it very likely that ANY of us would be inclined, in our self-centered nature, to blame God, to loathe Him for letting us feel such pain. We would be very unlikely, if it were even remotely possible, to admit that He was justified, in the midst of our agony. If we apologized to God, it would probably just be a ploy to get out of our pain, not an act of genuine love that recognized that our fate was just. And without the Sacrament of Penance, which is itself a mercy that we haven’t earned, only that sort of true contrition CAN breach the gap with God.
The souls in Purgatory have the Grace to come to that true contrition, in spite of the overwhelming pain, as by God’s Grace they are free of the self-absorbed tendency to loathe anyone who would allow you to undergo SUCH extreme pain even if they are justified in allowing it. The souls in Hell, however, have rejected that grace, and thus from the moment their torment begins, they are on their own. I highly suspect that, without supernatural Grace, while they hypothetically have the free will to choose God, to truly love Him and genuinely admit that He is justified in punishing them (which would be required for True Contrition, and which the souls in Purgatory freely would choose and admit due to God’s Grace), they simply
won’t do this. They will instead blame God for their fate, perhaps being compelled by an inability to lie to admit (in certain visions and apparitions) that their punishment is just, but always inwardly resenting it and only admitting it very grudgingly, not for love of God. And as long as this state is true, they cannot truly repent and enter Heaven. And without Grace, which they have willingly rejected or ignored when it was available, this state of being will be eternal, not because they’re “frozen” against their will, but precisely because they are never inclined to use their will to make a legitimate change.
God does not OWE them Grace; it is a free gift, one that He offered liberally in this lifetime. He doesn’t even OWE it to us in this life. I personally suspect that if a soul in Hell WOULD repent without God’s Grace, that soul would be saved, but I suspect that this just doesn’t ever happen.
So I think maybe it’s more of “It just isn’t a reality that anyone repents after death” rather than “impossible” in the sense that they couldn’t even if they wanted to do so. In a sense, if this is true, then it’s “impossible” in the sense that it just won’t happen, and God in His omniscience knows that it won’t and therefore it’s technically impossible that it will happen otherwise, but not because there will be souls in Hell who want desperately to repent but just can’t because they are somehow deprived, against their will, of choosing that option.
I hope this makes some sense, and of course these are my own musings; if any of them fall outside of Catholic orthodoxy without my realizing it, then I am definitely the wrong one, not the Church.
Blessings in Christ,
KindredSoul