About a year ago, a few months before she left sixth-form college, my youngest daughter asked cheerily: "What will you feel when you have no one left to wave goodbye to in the morning?" And as if someone had pressed a button, I burst into snorting floods of tears.
It felt like a sea of sorrow; I didn't know how I would ever stop. I didn't even know why I was weeping with such abandon: because she was leaving; because the other three had already left; because I missed them so; because I missed the person I was when they were all young; because their childhoods were over and had been happy; because I couldn't recover those days when I knew I could make them safe and protect them from the world; because I was scared of who I would be without them...
So begins the goodbye. I was appalled by an event I must always have known would come. I don't want them to stay; it's shockingly painful to let go.
I sometimes think I'm like scaffolding erected around a building and now the building has gone and just the scaffolding is left. Although I have always worked, since September 1987 when my son was born, the shape of my life has been dictated by my children, their needs and moods (there's a saying that's like a curse: "you're only as happy as your least happy child").
Sleepless nights, early mornings, bottles and bibs, nappies, potty-training, the small thrashing body in your bed, night terrors, dirty clothes, hot, cross, overcrowded cars, mashed-up meals, buggies, bath time, first days at school, scraped knees, tantrums, a warm hand in yours, nits, measles and colds on a loop, sandcastles, school concerts and parents' evenings, childcare and the regular collapse of childcare, the call at work to say they're ill, reading to them at bedtime, shouting to them in the supermarket, helping them with homework, lunch boxes, reports, exams, friendship problems, lost socks, lost PE gear, lost coursework, lost everything, banging doors, bedrooms that throb with mess, late-night calls asking to be collected, beer cans on the lawn, vodka bottles on the lawn, first romances, first holidays away from you, first festivals, first heartbreak, the gradual realisation that they have secrets, the gradual sense that you can no longer make everything all right, the endless juggle that is called parenthood and that you only realise when it's over is also, perhaps, called happiness.
And then, if things go the way you want, if you're lucky, they leave. I have been lucky and they've left - and like a machine evolved to process the daily churn of their needs, I continue spinning uselessly in their absence.
I have been quite taken aback by the strength of my missing, but also by how so many of my friends feel exactly the same, and how physical it is.
We talk about going into the empty bedrooms - the room whose mess we used to complain about - and about the days that were for years crammed with thankless domestic tasks and now have a kind of spaciousness about them. I have the time I longed for; I can read books, go for walks, see friends, grow chilli plants, paint badly, think about learning a language - but my mind hasn't grasped my new freedom yet.
When a tiny child calls for its mother, I still turn round. The heart takes time to catch up with change that feels like a cinematic jump-cut.
You're young and starting out and, all of a sudden, you're middle-aged: a crumpled, creased, pouchy face gazes in startled outrage from the mirror.
The problem is not that they go; it's that you stay behind, in a life that suddenly feels the wrong shape. The terrible story of rock star Georgie Fame's wife, Nicolette Powell, who in 1993 jumped to her death from a bridge after her children left, is an extreme example of how for many parents, particularly mothers, the transition can feel like a bereavement, a redundancy, a sudden loss of purpose and worth.
How to turn such loss into adventure and liberation? I know a couple who built a house together when their last child went; others who have gone on long trips, changed jobs, taken up new passions and learned new skills. It feels important to be reckless, selfish and young again - open to change.
For myself, I've been learning how to throw pots on a wheel and last year I trained to become a humanist celebrant. I can now conduct funerals, ritualising farewells, trying to help people to say goodbye to those they have loved. Yet in spite of my best efforts I still often wake in the night with a sense of heart-thumping dread. When the tide goes out, nasty things are found on the sand.
However, I also know that this empty-nest syndrome is a form of happiness. It's an ache of love, a good and proper sadness. And you don't really want them back. Never mind empty-nest syndrome - what about full-nest syndrome, just as problematic? This ache is not a real bereavement, though for a while the heart can be misleading. The children have grown up and gone, as they should and as you in your turn went - but they haven't died and they haven't gone missing. They're probably round the corner with their dirty laundry.