How can a good God punish someone forever? I find it hard to believe.
It depends on how you look at it. If you believe that hell is God continually punishing people and deriving pleasure from such, then I can see why it will be hard to accept. It is hard to accept since this isn’t reasonable since God is loving.
A better way to look at it is the damned do not want to be saved and do not want to receive help from God. While on earth, the person receives countless graces and inspirations to turn to God, but they refuse. At death, God respects their final wish to be “left alone.” They do not stop sinning even at death; since their souls are far from God’s light, all they can conceive for themselves is hopelessness and unjust anger towards God and creation.
So, they keep relighting the fires of their own hells with hatred and other countless sins they dwell in for all eternity. I have always believed that if a soul in hell were to lift their hearts to heaven and ask for mercy sincerely, God would respond. This is impossible, however, not because God wouldn’t forgive but because the souls in hell would never ask for mercy.
A person who has spent most of his life loathing God’s presence and choosing self-love will no doubt hate to behold or be in God’s presence where everything is made clear. I feel like this has everything to do with love and truth. Dwelling in darkness makes the light unbearable to the eyes already accustomed to the “ease” of darkness and runs from the revelation and initial harshness of the light.
I recommend reading this article too.
And this piece from Catholic answers
How can God punish someone forever?