Everyone knows exercising is a mood-booster, but not only is it hard — it can also be expensive.

Which is why it’s fantastic that recent research from the UK indicates there’s a straightforward (and free!) method to adjust your attitude — though there is a slight catch.

The study — published Tuesday in the journal PLOS One — suggests that so-called “positive expressive writing” can seriously lift your spirits.

Instead of pouring out all your frustrations, positive expressive writing emphasizes gratitude, self-reflection, and visualizing a bright future for yourself.

This marks a significant departure from the early approaches to therapeutic writing, where participants were urged to continually revisit trauma and stress — a technique the study authors admit might be beneficial over time but could deepen your immediate sense of despair.

The upbeat version, by contrast, skips the tears and gets right to the feel-good stuff.

Having analyzed 51 studies spanning from 1930 to 2023, however, the researchers admit positive expressive writing wasn’t a slam dunk for everyone.

Some folks flourished — others didn’t budge.

Why the mixed bag? It could come down to the way the writing sessions were run — or to the personalities putting pen to paper.

Researchers say more standardized protocols and closer attention to individual differences are needed before journaling can be billed as a universal fix.

That being said — these findings do align with an increasing amount of research that shows maintaining a positive outlook on life and practicing gratitude are some of the easiest happiness hacks.

“Gratitude is mentioned in almost every conversation of happiness and with good reason — it is an emotional trump card,” mindset and meditation expert Joanna Rajendran previously told The Post.

She recommends starting small.

“While things like your health, your home, and your family all may be accurate answers, if said so generally or so often, they will no longer have the same emotional impact,” she said. “Instead, look around where you are right this moment and start with the simple pleasures and begin to list them.”

Experts also often list journaling in general as a great way to reduce everyday stress, as well as “anxiety detox” before bed.

In one oft-quoted study, people who spent just 15 minutes per day journaling felt significantly less anxiety, depression and overall distress.

So next time you’re feeling down — remember that the pen just might be mightier than the Prozac.