My husband and I don’t think we have the condition until, one day last month, hundreds of miles from home, we find ourselves outside our younger son’s university accommodation at 11.30 on a Sunday morning. I am clutching supplies in a little brown paper bag. Our son knows we’re in town, but isn’t expecting this rude awakening. It’s a surprise.

“Do you think we should have called first?” I say as we approach the entrance, the inappropriateness of what we’re doing dawning on me only now.

My husband pulls a face; it’s his moment of awakening, too. “Call now,” he says. “Tell him we’re on our way.”

“But we’re already here.”

We both suddenly see what we have become: two newbie empty-nester weirdos. For anyone to pitch up at an 18-year-old’s gaff on a Sunday morning is never a good idea. For the parents to do so is intrusive and insensitive. I call anyway. We’ve rustled up two blueberry muffins and an apple from the hotel breakfast buffet and picked up some fresh orange juice. Plus, we’ve spent 20 minutes walking here. And he needs to know we care.

At first he does not pick up. When he does, he says in a sleep-strangled drawl: “Maybe could you just go away and come back later.”

Fair enough, even quite polite, in the circumstances. But I say: “Well, actually, we’re already outside. Maybe you could just nip to the door and then go back to bed?” Cue a heartfelt groan. A few minutes later a hand appears through the opened door. The bag disappears.

So, it’s official then. We have Empty Nest Syndrome (ENS). Since our second son left for university in September we have joined “the left behind”, to coin a spooky-sounding term from a 2018 study of this non-clinical condition, conducted in China. It sounds like a six-part TV drama: One empty nest and two parents who will stop at nothing to clip their children’s wings.

China takes ENS very seriously. It's predicted that by 2030 there will be more than 200m empty-nesters

Episode one starts this Christmas with me pimping the nest within an inch of its life so that the boys weep at the downy, desirable thing called home and never want to leave again. That was the plan until, earlier this month, two different friends poisoned my ear within 24 hours of each other, casually remarking that this might be the last time my sons came home for Crimbo. Sorry, what? They might choose to have it elsewhere in the future, one said breezily, or with friends or squeezes, said the other.

I am fully aware that children leave home, with some boomeranging back when it suits. I also know that parenting is a process of letting go, and not just of a disposable income. Migration is natural. But the idea that a cuckoo might entice my sons away with a more attractive festive roost makes me defensive, primally so. Plus, it ramps up the pressure to put on the best Christmas ever, and I already do a good one. But piling on the bling is a no-no, the end-of-childhood bell having already been sounded by my sons banning Poundshop junk, plastic and “humorous” gifts, such as wind-up Trump figurines, tabletop golf and novelty socks. This year, stockings must contain sustainable, sophisticated objets. Quality over quantity is the watchword. All very grown-up – and there’s the rub. I appear to have joined what other ENS studies refer to as “midlife parents”, “empty nest older adults”, and “the elderly whose children have left the old adults alone at home”. The elderly. The old adults. No wonder I feel washed up. Time has been called on the chief focus of my existence for the past 20 years.

It is predicted that by 2030 there will be more than 200m empty-nesters in China, a country that takes ENS very seriously, partly because the one-child policy implemented in 1980 (and terminated in 2015) means that for a vast swathe of empty-nesters there isn’t even the salve of a nest half-full, a period of transition in which the children leave one by one, supposedly softening the blow. I’ve had this, thanks to a two-year age gap between my sons. It hasn’t made their launch any easier, possibly because I thought I would be glad when my second son “migrated”. I looked forward to more time for myself. But I hadn’t factored in the other bird in my nest: the husband.

Forget all that indulgent “me time”, the empty nest, chirp the experts, is a chance to “reconnect” and “rekindle” your relationship. Ours is very satisfactory as it is, thank you very much. But no, we must plan adventures together, say husband-and-wife marital wisdom podcasters Ashley and Marcus Kusi, authors of Our Bucket List Adventures: a Journal for Couples. Hmmm. Bucket lists, with their implications of the Grim Reaper metering your time, are not for me. There is no way I am going to invite my husband to celebrate our mortality à deux.

Since our sons have left, the fridge has been empty

“Repurpose yourself” is another common recommendation, which makes me feel like upcycled furniture. Kim Smith, an American counsellor and author of Embracing Next: an Empty Nest Enjoyment Guide, says it’s time to focus on the future. She’s keen on the “re” prefix, too. “Reframe, refocus and refeather” is her mantra for “maximising your enjoyment of the next chapter of life”.

US holistic psychotherapist Dina Molina, author of Empty Nest, Sexy House, talks of the “sexy power” that is ours for the taking. Go out, she says, have fun with friends, renovate your home. Erin Marshall, a US interior designer, actually specialises in EN renovations. Avoid turning your house into a shrine by filling every wall with photographs of your kids; curate them on a single wall instead, she advises. After all, she implies, you don’t want to look desperate.

But maybe we are. A friend tells of how he recently traipsed across his son’s town one sodden morning carrying bags of groceries for him. “I felt like a bird taking worms to the nest,” he tells me. Doing so sated his need to nurture, feed and provide, a need that he said can feel like a craving. Another parent has arranged an overnight business trip to her child’s university 300 miles away so that she can fill her daughter’s fridge, as she puts it, the night before her exams.

I wish she’d fill my fridge. Since our sons have left, it’s empty. It used to be bursting with food – meat for the carnivore, fish for the pescatarian, beers in case mates came over, a freezer full of pizzas for late-night munchies. I find it ghoulishly fascinating that, having nurtured our two sons for two decades, I now seem unable to feed, let alone nurture, myself or my husband. I have to force myself to buy groceries and remind myself it’s not fair to leave the husband to do all the cooking. On the upside, it is very nice to be waited on.

I catch myself wanting my currency as a mother affirmed, which feels weird and needy

At least we don’t cry during meals. An artist friend told me that when the eldest of her three children left home, she and her husband sat at the supper table with tears streaming down their faces, for weeks. So did the younger daughter, aged 16, and their son, 11. “My daughter missed her big sister,” says the mother, “and said she had lost her best friend. We were desolate.” The mother went into therapy for six months.

Grief is one of a litany of symptoms that includes sadness, loneliness, anxiety, restlessness, guilt, bodily aches and pains, anger, irritability and frustration. A dip in self-esteem is another which, compounded by a loss of social connections and a change in identity, is likened to the feelings experienced by the unemployed. For my part, I find myself questioning what, if anything, I can still offer my children, beyond love, a roof and a nice line in banana muffins; worse, I catch myself wanting my currency as a mother affirmed, which feels weird and needy. As ever, parenting is not about me, alas, or at least is not supposed to be.

If only we didn’t live as long as we now do, or involve ourselves in our children’s lives as much as we do. Back in 1900, short life spans and children living at home meant the empty nest only lasted two years. In the 1970s, when sociologists first popularised the term empty nest, it was 13 years. Today, with male and female life expectancy now at 79 and 83 respectively and the average age at which adult children fly the nest for good being 26, we’re talking decades – and that, I realise, still gives me plenty of Christmases to gild the nest.

Milkshakes and Morphine: a Memoir of Love and Life by Genevieve Fox is published by Vintage at £8.99. Buy it for £7.91 from guardianbookshop.com