The doctrine of Hell is essentially about freewill. Man can choose to embrace God for all eternity or to reject him, to say ‘yes’ or ‘no’, we can choose between life and death. In a very real way we are made like God himself, we are eternal.
On the Cross God shows himself powerless but we are never powerless, God removes his choice by choosing to love us. God accepts us with utter love and we are called to make a decision of accepting or rejecting him. Catholic theology never teaches God or his Grace is irresistable. In each of our acts we can either reject or accept God.
We can be in the crowd which shouts Hosanna – ‘Save us’ as we follow Jesus in the great sacrificial procession from misery (Bethany means ‘House of Misery) to be at peace with God (Jerusalem means ‘the View of Peace’) or we can be in that other crowd which rejects him and shouts “Crucify him” and does crucify him.
The choice is eternal because we are eternal, we can choose God which is Heaven or Hell where the pain is our own decision to reject God.
Heaven and Hell are both a single moment but an eternal moment.