As a culture, we are obsessed with ageing. We have always been obsessed but now, paradoxically, in an era where we live longer than ever, we fear it more than ever before too. There is, of course, a whole industry devoted to capitalising on our fears of the natural ageing process and it’s a lucrative one. In fact, the anti-ageing industry, which is the largest part of the beauty industry, is now worth more than $200 billion a year.

Our perfectly understandable worries about time and mortality are being capitalised on, and exploited. Every advert that encourages us to look young is confirming the same thing: we need to fear growing old. And yet no anti-wrinkle eye cream in the world is going to stop us from getting old. The anti-ageing industry is a marketer’s dream because it is an industry offering continual solutions for something that isn’t ever really solved. Ageing.

Even if or when we work out how to stop the physical process of ageing – and organisations such as the SENS Research Foundation and various Silicon Valley biotech firms are trying to do just that – it won’t curb our anxieties. It will accentuate them (not least, the ultimate fear of missing out for those who can’t afford it), and give us many new ones (a new population crisis being the first one).

In How to Stop Time, my novel about a 439-year-old man who ages far more slowly than normal, I wanted to think about what being really old would be like. I wanted to explore a character who was struggling with his relationship to time. To see if it would be possible to embrace the present when you had so much past and so much future. I wanted to look at how time does not always feel like the same thing.

I first truly understood this when I became ill with an overlapping cocktail of mental health problems in my 20s. I developed panic disorder, severe anxiety and depression, and it was well over a year before I began to feel anything like normal again. In my mind, that year felt like an eternity. Days stretched out for what felt like weeks – weeks for what felt like years. In my mind that year still feels like half a lifetime. Time was the enemy, in that sense, but in another way it was a friend.

You see, depression and anxiety told me a lot of lies that time could disprove. It told me I wouldn’t be alive to see my 25th birthday, or that by Christmas I’d be confined to a padded cell wearing a straitjacket. Time was the one thing bigger than depression itself. And I could feel its power as I built up days and weeks and months. It was the currency I traded in and accumulated. I have been ill for six days. I have been ill for 22 days. I have been ill for 365 days – and I am still here.

Eventually, very slowly, my mind readjusted itself and worked out the lies that depression and anxiety were telling it. The oldest cliché in the world – time heals – turned out to be a cliché for a reason.

So, I have an ambiguous relationship to time. I do have worries about growing old, like everyone, but I would have equal worries about not growing old. Rather than searching for a magical elixir of eternal life, we should understand – like Emily Dickinson did – that “forever is composed of nows”. The challenge is in how to stop worrying and inhabit the nows that we have.

How to Stop Time by Matt Haig is published by Canongate Books, priced at £12.99. To order a copy for £7.99, go to bookshop.theguardian.com