Waking up today, on so-called Blue Monday, the “most depressing day of the year”, you may already be aware that this concept is based on a fraud. An almost deliciously spurious mathematical formula was dreamed up by a PR agency, given the veneer of academic rigour by attaching the name of a lecturer at a further education college, and a media phenomenon was born. More than a decade’s worth of articles and social media memes have at turns reinforced and defied the Blue Monday myth, and it is often now held up as a case study of bad science.

It is, of course, laughable to have a formula where W stands for weather and days-since-Christmas is raised to the power of Q, the days-since-we-quit-our-new-year-resolutions. But the driver behind this mockable maths is a much more sinister lie, one from which many struggle to escape 365 days of the year.

Blue Monday was originally invented for an advertising campaign for Sky Travel. Its sole purpose was to sell holidays with the false promise that spending money would raise flagging spirits. Blue Monday has now been used to sell everything from flowers to Ferraris, takeaways to airport parking. Beat Blue Monday, they tell us (laughing knowingly at the fact that it’s made up): buy something new today.

We’re told every day by advertisers that buying stuff will make us happy. A new pair of shoes will help us to feel better after a breakup. New beauty products will give us a sense that we’re “worth it”. A bigger car will give us social status. New toys will make the children happy. Even loans are sold this way: one payday lender is currently running a campaign with a smiling woman, snuggling a mug of tea and feeling happy thanks to a 1,200% APR loan, an implausible scenario if ever there was one. So it’s no wonder that on Blue Monday, the day our anxieties and misery are supposed to peak, the advertisers scream that the path out of unhappiness is paved with till receipts.

Sometimes the harm this causes is relatively trivial: overconsumption of stuff we don’t need; a bit less money in the bank; a wardrobe that won’t close because we’re not very good at throwing away our throwaway fashion buys.

But the harm can be deadly serious for people struggling with mental health problems. Imagine it: your mood is low. You find yourself crippled by anxiety. You feel like a failure, a burden on your family and friends. This is the moment when a lifetime of being told that stuff will make you feel better takes its toll: in desperation, nine in 10 people with a mental health problems find themselves spending more when they’re feeling unwell.

In my work at the research charity Money and Mental Health, I’ve lost count of the stories I’ve heard that stop me in my tracks. Mothers spending their way through months of postnatal depression – one who bought nearly 100 buggies over four years. A man who tried to buy a villa in a country he didn’t have a visa for, convinced it would turn his life around. A young woman who found buying something online was the only way she could stop a panic attack, so she did it almost every day. A husband who felt such a burden he’d buy endless presents he couldn’t afford for his wife. And many of them tell us that the boxes sit in the living room, unopened, unwanted, but impossible to ignore.

Because buying stuff is just the first part of the cycle. Next comes the guilt. People with mental health problems are far more likely to be living on a low income, and the financial damage that compulsive shopping can do is extraordinary. The boxes – or if they’re actually opened, the new possessions – are a constant reminder of the mistakes made. Too often, people don’t return unwanted goods: nearly half tell us that’s because they just want to pretend it never happened. The guilt brings their mood lower and then buying something for a temporary buzz feels like the only way back up.

We have just launched a new tool, the Shopper Stopper, which is helping people to curb their night-time shopping in particular by allowing them to set the opening hours of online shops. But it’s going to take a wider societal shift to really shake us out of these habits for good.

Comfort eating and substance abuse – other common ways to medicate mood – have physical effects. But the psychological impact of new stuff comes from our culture, not our chemistry. We can, if we change the way we talk about belongings, change the way we all think about shopping. And the people who would benefit the most are those struggling every day to find enough happiness to stay alive, and driving themselves into debt on a doomed quest to find it in a shopping basket. You cannot buy happiness – not even on “Blue Monday” – and the sooner we destroy the myth that you can, the happier we all will be.