Greetings, Glum-Of!
There is disputation to this day about the doctrine of "eternal security"--the belief that once we say the words "I believe in Jesus," we have a guaranteed free ticket to Heaven NO MATTER WHAT we do. But if accepting Jesus as our Savior does not mean eternal security strictly in this sense, it's as close to it as we need. Even if it IS possible for us, after gaining a relationship with God, to lose it, God will never make it EASY for us to lose it. He is FOR us, not against us. He wants us to succeed, not to fail. Thus the famous Parable of the Prodigal Son: the title character of this parable in Luke's Gospel had offended against his loving father VERY badly, treating the father with contempt and dishonoring the family name--but the father still was not only willing to forgive him, but DELIGHTED to have the relationship restored.
The fact that "The Great Divorce" metaphorically shows some lost souls being saved AFTER death--not something we should count on in reality!--reflects Mr. Lewis' understanding of God's merciful nature. Even in the Old Testament, before Jesus had come to pay for our sins, God said to the prophet Ezekiel, "As surely as I live, I take no pleasure in the death of the wicked, but rather that they should turn from their way and live."
So you can have, not only a forlorn hope, but a strong confidence, that God is on your side--that He will be at work to HELP you toward Heaven, rather than planting landmines to PREVENT you from getting there.
Oh, while I'm thinking of it: in spite of the atheistic leanings of the "Doctor Who" series, there must have been someone among the writers who had read "The Great Divorce"....because the Great Lady is named Sarah Smith, and lived on Earth at Golders Green, England. The Doctor Who character Sarah JANE Smith was once referred to as living at Golders Green also!