This is the season of goodbyes. At the weekend I visited perhaps the most beautiful museum on earth, the Louisiana art gallery in Denmark, and stood in the sculpture park next to the beach, watching the sailboats bob across the water. It was a perfect day, and its perfection made me unhappy. It was a ready-made memory: the last day of summer, 2015.

Still, I knew it wasn’t just the fading sunshine that was making the day so bitter-sweet. On the train out of Copenhagen, I had started to read The Shepherd’s Crown. It is the final Discworld novel; its author, Terry Pratchett, died of early-onset Alzheimer’s on 12 March, leaving behind dozens of brilliant books, and dozens more left unwritten. (His assistant Rob Wilkins notes in the afterword that “we will now not know how the old folk of Twilight Canyon solve the mystery of a missing treasure and defeat the rise of a Dark Lord despite their failing memories, nor the secret of the crystal cave and the carnivorous plants in The Dark Incontinent . . . and these are just a few of the ideas his office and family know about”.)

Pratchett was diagnosed with the illness that killed him in 2007. He called it “the embuggerance” and set about making every remaining day count. He wrote books even when he could no longer write, dictating them to Wilkins, and became an impassioned advocate for euthanasia. He wanted to die in his garden, he said, drinking a brandy, with Thomas Tallis on his iPod. (“Oh, and since this is England, I had better add, ‘If wet, in the library.’”)

Since March, I have been reading the few remaining Discworld books I never tackled during Pratchett’s lifetime. I had never got round to reading his series about the junior witch Tiffany Aching. Shamefully, I think I saw “young adult” and my inner dowager duchess reached for the smelling salts.

That was my stupid mistake. The Aching books are some of Pratchett’s best, and I fell so instantly in love that I had a passage from one of them at my wedding this summer. So The Shepherd’s Crown was a double sadness: not just goodbye to Terry Pratchett, but goodbye to new adventures for Tiffany Aching, to Nanny Ogg, to Greebo the smelly, one-eyed tomcat and to Magrat, the drippy hippie queen who nevertheless shot an elf in the eye with a crossbow through a keyhole when her friends were in danger.

Most of all, it was goodbye to Esme Weatherwax, who dies right at the start of the novel. Like all witches, she gets some advance warning – in her case, the premonition comes as she’s cleaning the privy. She spends her last day scrubbing her tiny woodland cottage from top to bottom, choosing a spot for her grave and weaving a makeshift coffin from switches of willow. And then she goes to her bedroom and dies.

The quietness of it is what punches you. Like real deaths, there is no spectacle. It’s not freighted with meaning. It doesn’t function as a major plot point. It just is. And everyone else just has to go on.

If you haven’t read any of Pratchett’s books, it is hard to explain what Granny Weatherwax represents. She is probably the closest thing I have to a religion. She believes in hard work – delivering babies and clipping widowers’ toenails is a larger part of being a witch than using magic – without seeking glory or material reward. Like Samuel Vimes, Pratchett’s other great moral hero, she is unyielding (her nickname among the dwarves is Go Around The Other Side Of The Mountain) and immune to bribes or flattery. She is not without ego or pride but is always determined to do what is right, not what is most pleasant or easy. She is stubborn and austere and lives alone, but that is the price of doing what she does.

And I love her. I love her wholeheartedly, and without a wisp of the usual cynicism I would reserve for anything or anyone who is too good to be true. I love Terry Pratchett, too, and have done since the moment I picked up Mort, his fourth Discworld book, on a rainy holiday in Brittany two decades ago. His world-view has always been so humane, patient and forgiving, without ever lapsing into permissive do-gooderism or pessimistic libertarianism. Reading his books made me love the human race.

And that is what I was really saying goodbye to, as I snuffled quietly to myself on the train, surrounded by strapping Danes on a day trip to the countryside. I’m never going to love another author like I loved Terry Pratchett, because that love was born of being 13 and fat and lightly bullied, and because the internet these days is just a giant machine for telling you what’s wrong with the things you like. (There’s a dispiriting Tumblr called Your Fave is Problematic. Spoiler alert: all your faves are problematic.)

Scepticism is healthy, but cynicism is corrosive. And yet the tone of modern life is overwhelming cynical – how could it not be, when enthusiasm feels so uncool and criticism is so easy? Just as a pessimist is never disappointed, a cynic is never humiliated by the crushing of a deeply held belief.

I’m not exempting myself from this criticism. Just before I left for Denmark, I went to a Jeremy Corbyn rally at the Union Chapel in Islington. I felt like the only atheist at an evangelical church meeting. Everyone else seemed . . . happy. Uncomplicatedly, straightforwardly optimistic. Meanwhile, I was sitting at the back drafting snarky put-downs for when I retold the story.

Perhaps those people are doomed to disappointment over the next few months (although knowing all the words to “The Red Flag” suggests a certain resilience). Perhaps all the confident naysaying is right, and Corbyn will be a disaster. But still, his supporters will have experienced something I don’t think I ever will again, by daring to believe in a cause so unreservedly and wholeheartedly. Now that Terry Pratchett and Granny Weatherwax are dead, I worry that I will never again be more than a cynic.