Nostalgia, I’ve come to realise, is a trap and a tease. It hoodwinks you into believing that a past you can never reclaim is better than your present can ever be. The act of remembering, on the other hand, can be liberating, provided you’ve wised up to just how capricious our often airbrushed, invariably unreliable, memories can be. It took me a while.

I’ve always had a bad feeling about nostalgia, though. It is nationalism’s bedfellow, for starters, and it makes us wish interminable, internet-free holidays, three-channel television and the warty good old days upon our children.

When I wrote my memoir, an unlikely act for someone who had spent most of her life doggedly not looking back, I finally saw how nostalgia had held me and my two older siblings in its grip. The memoir is a story about my childhood misadventures after being orphaned to cancer, aged nine. Our American father dropped dead when I was five, after which our English mother brought us to the UK. After her death from cancer four years later, we had a succession of carers. I held my breath, waiting for everything to come right. Godot comes to mind.

I only revisited what my older son now calls a series of unfortunate events because I went and got cancer myself, which in turn threatened to make my teenage sons motherless, a threat that triggered old memories: my period of illness was peppered with involuntary flashbacks to my mother’s own dying and to a landscape of perceived abandonment. The writing was my way of taking dominion over these flashbacks of love and loss.

Take your memory by the scruff of the neck so it quivers with fear

It enabled me to pause, freeze-frame, rewind, zoom in and turn up the volume on the memories that had come at me like shoppers charging through the doors at a Black Friday Primark sale. I saw that what we had before both parents vanished was a Disneyesque blueprint of how our lives should be.

We didn’t like our post-parent new life, one lived with strangers and half-here, half-not relatives. We wanted our old one back, thank you very much. So we constructed celluloid versions in our minds, and a physical one in the approximation of a home we eventually created for ourselves in London after the long-suffering, insufferable woman who had looked after us for three years jumped ship when I was 13, in the summer of 1977.

Some may remember that as the summer Elvis Presley died. I remember it as the arid one where we unpacked tea chests and quickly laid out the family collection of fox figurines on the new mantelpiece, just as our mother had once done. There was a vulpine conductor and a violinist, part of an incomplete orchestra and, my favourite, a pair of dancing foxes. It was our shrine, to the good old, rose-tinted days, to a time of a father and a mother dressed up in white tie and cocktail frock, a family unit, a perfect life.

The mementoes made it harder for each of us to let go of a past that became increasingly mythologised and a future that felt increasingly wanting. They’ve come to symbolise a Miss Havisham-like difficulty in moving on. Nothing could ever measure up to the chimerical promise of what might have been.

The trouble is, I couldn’t remember the details of this idealised, longed-for past life as well as everyone else seemed able to do. My older siblings, and any adults who knew the dead parents I sought to conjure in my mind, had “better” memories than me. They corrected me when I said I remembered this or that. No, Father wasn’t left-handed. No, that’s not what our cat was called. No, Mother didn’t call lavatories “loos”. I came to see memory as a monolith, a slab of granite, fixed and formidable. I avoided it.

Bear in mind that it was not until the early 90s that neuroscientists established the mutable nature of memory that is now a given. We know that each memory is a reconstruction, an aggregate of each and every previous memory at the time of retrieval. But I had grown up believing that a memory was intractable: that what we remembered was the original memory. For me, not having certain memories, or not remembering “as well” as my siblings, was humiliating. The fact that, secretly, I couldn’t remember much about my father, bar him making me toffee for breakfast in a frying pan (an unreliable memory, if ever there was one) and him sitting me on his knee as he did up the buckles of my patent Mary Jane party shoes, made me think I did not really ever have a father. I let my older siblings and half-siblings recall him, and claim him as their own.

Has something ever been yours if you can’t remember it?

I came to see myself as a person with a bad memory. That changed with the memoir. In writing it, I took memory by the scruff of its neck. I backed it up against the wall, watched it quiver with fear. I recommend doing so. Flawed, pockmarked, crusted in layers of transmogrified facts and fictions, memory when looked in the face is like an Elizabethan courtier plastered with a mask of white lead, revealing the cracks for the first time. Its power falls away. Now I think, It’s OK, I forgot something – the scale of a childhood house, the age of my aged father when he had me. Who cares?

Note what you seek to recall and you will see what you really value

For sure, the impulse to fill in the gaps, to complete the jigsaw of your past, is a universal one, but what is most important is the emotion an episodic memory contains. That emotion contains the essence of what matters to you, which is the very first step in what is remembered – the essence of what is past. Take a note of what you seek to recall – that time when you felt safe, rather than afraid, say; or that afternoon you laughed with a friend until your belly hurt – and you will see what you miss, need, value. And you can seek out more of these positive emotions in the future. Remembering, and imagining the future, are part of the same mental process. Actively remembering is a far cry from nostalgia; there is a freedom in it.

Of course, there is a danger in remembering, too, particularly for those who have suffered a trauma. The more we recall a memory, the stronger it becomes, so that is problematic when it comes to traumatic memories.

I recently met a psychologist who practises Narrative Exposure Therapy at the Helen Bamber Foundation with survivors of human cruelty – those who have been trafficked, abused, sexually exploited, enslaved, and suffer from PTSD. In this pioneering form of therapy, clients are invited to remember the good as well as the bad things from their lives, represented by flowers and stones respectively. A timeline of their life is represented by a ribbon or length of rope, and as they remember, they are invited to lay down a stone or a flower, sharing each memory aloud as they do so.

It is a way of acknowledging and speaking the unspeakable, as well as of remembering who you were before the trauma, or multiple traumas, were inflicted. Little by little, the client integrates their traumatised self within the bigger narrative of a life that need not be defined only by the bad things. Many are thus enabled to move forwards. They can imagine a future.

Nowadays, I let myself remember like there is no tomorrow. I dig for gold, and relish the finds: my mother dressing up for my mocked-up marriage to my teddy bear; my sister and me playing under a sprinkler in the park after school; the honeybee joy of being loved. Mining those feelings is a way of picking and choosing the building blocks of the person I want to be from now on. I claim these memories, not as half-formed things, or inaccurate composites of hearsay and photographs and other people’s memories, but simply as my memories.

The act of remembering is a legitimate, positive thing to do, not a weakness, as I used to think. I mean, it was good enough for Proust, and painters do it, time and again. Take Pierre Bonnard, whose exhibition, The Colour of Memory, has just opened at Tate Modern. He captures what he cherished – concrete things like a house or a garden – in increasingly abstract, vibrant paintings. He captures the feeling of what he loved. I urge you to do the same.

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