Lotta Sonninen was working on her third book in quick succession on positivity, when she suddenly realised her own positivity had drained clean away. “All those invitations to perk up, smile, find the happiness in everything were getting on my nerves,” she says. “I thought, hang on a minute. I’m sick and tired of finding the good in everything. What if we started to play with the opposite of all this – our negative emotions, the feelings we all have inside us that we’re not being encouraged to share? I thought, I wonder what’s happening to these emotions – and could we do something with them more useful than burying them?”

She started making lists that were the antidote to the mindfulness and positivity books she was translating: so instead of listing what she had to be grateful for, or what was lucky in her life, she turned to all the negative stuff in her life. All the angst and agony; all the silent anger and pent-up frustrations. How about a list of all those acquaintances and celebrities who’d obviously had it far too easy? And next, what about naming all those arrogant folk who thought oh-so-much of themselves? And those people who seemed not to have clocked that you really need to think before you open your mouth? And how about writing down the names of all those infuriating people who were clearly far less talented than her, but somehow way more successful?

After that, she turned to social media. What were the comments that made her want to throw her phone against the wall? She trawled Facebook for the most infuriating status updates; surfed Instagram for the stupidest comments; scoured Twitter for the most birdbrained tweets. Next, she mused on which emojis really deserved to be punched in the face and, in fact, what new emoji should she design, one that did justice to her new-found anger, rage and frustration. How come so many emojis showcased cheeriness and buoyancy, and hardly any were designed to reflect the important emotions of malaise, gloom and despondency?

“It started as a private joke: a counterstrike against all that upbeat positivity,” says Sonninen. But as she tentatively began to share her secret negativity project with friends in Helsinki, where she’s a book editor and translator of texts from English into Finnish, it was clear she had tapped into a rich seam.

“All that ranting and raving we found ourselves doing was so cleansing and purifying,” she says. “We began to realise what a relief it was just to acknowledge our negative feelings. Often they felt so petty, but it was still good to get them out there.”

The smallness of what upset me really made me laugh

And yet, she figured, maybe it wasn’t all that surprising: Finland’s dark winter days (when we speak it’s midday in Helsinki, but still barely light) help explain the culture of appreciating the noir side of life.

“There’s a lot of pessimism in Finnish culture,” she says. “We love to revel in a bad mood, and we definitely don’t spend enough time discussing our feelings.”

So Sonninen wrote what she describes as an anti-mindfulness book, The Little Book of Bad Moods (sell: “Find the misery in every day and let go of nothing”) thinking it would have some appeal in her native country. But what soon became clear, she says, is that gloom is universal, and all of us want to vent our gripes, at least some of the time. The book has now been published in 30 countries and rising, with Indonesia, China and Japan among them. And while she says she wouldn’t go so far as to suggest you actually do the exercises the book describes, she sure as hell thinks you’ll feel liberated just imagining them: we’re talking, for example, about pages with space to write down ways to irritate your partner; tactics to upset your colleagues; strategies to be annoying online.

It also turned out that, like all the best jokes in life, there was something very serious at the heart of what Sonninen was exploring. “I don’t want to be preachy,” she says. “But I noticed that when you focus on the negative, it’s easy to blame others. Because you can always blame someone other than yourself, for anything in your life.”

And while that can be genuinely cathartic, it’s not usually a key to self-improvement beyond that. “The book stays a joke if you want it to,” says Sonninen, “but if you allow this, it can turn into an exercise of real self-exploration,” because we all know there’s only one person we can ever truly change in life – and it’s not someone else. The book can also, she promises, prompt a huge amount of fun and laughter – and the person you’re going to find yourself laughing at will be yourself. “It’s funny how much we sweat the small stuff, and how much we let other people annoy us. As I started to make my lists and think about my negative feelings, the smallness of so much of what upsets me really made me laugh,” says Sonninen.

Stuff from my teenage years had sunk deep and stayed there

Another thing that played on her mind was that all the mindfulness/creativity/positivity books currently flooding the market are aimed mostly at women: women who surely already have more than enough to do, without being required to squeeze mindfulness and positivity into the slender crevices of their day. “It felt like another duty – something else that was required of us, on top of everything else.” She certainly knows what she’s talking about on that score: as well as being a successful fiction editor, Sonninen, 43, has translated books including Fifty Shades of Grey and Lena Denham’s Not That Kind of Girl. On the home front, she’s mother to three-month-old twins, Voitto and Taisto; and just to make things busier still, she also plays in a band, and does the occasional bit of stand-up comedy. So, yes, mindfulness: bring it on, in that five-minute window she gets once a fortnight.

One thing Sonninen was surprised about, as she mined her memories for people who had wronged or upset or injured her, was how painful those events still felt – even decades down the line. “I hadn’t expected how much old things would turn out to be still bothering me – stuff from my teenage years that had sunk deep and stayed there.” Perhaps, she mused, we need to revisit these painful situations, instead of continually brushing over them in our constant quest for all that’s good and wholesome and positive. There may be things you still have to say to that ex you think is a bloodsucking vampire, or the teacher who humiliated you when you were seven in front of the whole class, or the friend who chose to ghost rather than support you. These hurts go deep, and the pain lies dormant until we pull them out of their box, shake them down and feel all those feelings once again, before – hopefully – letting them go. “I think giving your feelings a name helps: once a feeling has a label, you know what it’s about and why it’s there. That makes it easier to cope with: and then you can set it aside, and even move on from it.”

How to channel your bad moods

• Buried negative emotions can be stressful: you can channel too much of yourself into keeping them in check or repressed. It might feel as though avoiding anger or negativity will stop you feeling those things – but it takes a lot of effort to avoid them, and that’s effort you could better direct elsewhere. And anger can be helpful: it can certainly help you understand yourself better, because by thinking about what upsets or annoys you, you become more aware of your personal values.

• Acknowledging these feelings, and feeling what they mean us to feel, can also allow you to let them go and move on. All feelings are there for a reason: the important thing is to decode the feelings, to see if there’s a useful message to take from them, and if there is, to think about it. Frustration, anger and anxiety are all feelings that point to a need for change.

• Take a leaf out of Sonninen’s book and write down what’s most annoying you right now (don’t worry, you can destroy the piece of paper afterwards). Is it your partner? Your kids? Your parents? A friend? A colleague? Why are you annoyed with them? What’s most irritating about the situation? How does it make you feel? Have a cup of tea, then read what you’ve written and see whether this helps you decipher what you can do to change the situation. But remember: there’s only one person it’s ever going to be possible to change…

The Little Book of Bad Moods by Lotta Sonninen is published in the UK by Bloomsbury at £5.99. Buy it for £5.27 at guardianbookshop.com