Nobody expected Bernadette Joy to shut down her successful business. Not even Joy herself.

Joy ran Charlotte, North Carolina-based Dressed, which started as a side hustle connecting people who owned bridesmaid dresses with people who wanted to rent them. It helped her pay off her student loans, quit her full-time job, hire employees and open a storefront.

But the more momentum Dressed gained, the more time-intensive and less fulfilling it became. In 2019, after three years, she closed up shop. She'd also gotten a taste of being her own boss, and she didn't want to go back — prompting an "existential midlife crisis," she says.

"I spent [three months] wrapped up in my blanket on my couch thinking, 'What am I going to do?' because I didn't have a business anymore," says Joy, 39.

On that couch, she learned an important lesson, which she says is her No. 1 tip for starting a side hustle: Try to monetize what comes naturally to you, instead of selling something just because you can.

DON'T MISS: The ultimate guide to earning passive income online

In her case, that was her other side hustle: a podcast about how she and her husband were paying off more than $300,000 in debt. The show grew into a money-coaching business called Crush Your Money Goals, which helped Joy make $279,000 last year, or roughly $23,250 per month on average, according to documents reviewed by CNBC Make It.

Between coaching, freelancing and booking speaking events, she works 20 hours per week, she says.

Hard-earned lessons about passion

Joy wouldn't have launched Dressed if she wasn't passionate about fashion. But dealing with the complications of scaling a business didn't feel natural to her, she says.

She felt discouraged when dresses came back damaged: After prom season in 2019, for example, many of them came back torn and stained, she says. Joy had to buy the totaled gowns from their owners.

Her interns struggled without supervision, Joy adds, meaning she had to spend more time in her storefront, a converted office space just above a dentist's office. "I couldn't see myself in retail working all those hours," she says.

Attending personal finance conference FinCon that summer was the final straw. After listening to speakers, she knew she wanted to help other people feel more confident about their finances — even if she didn't quite know how yet.

She kept recording her podcast while researching how to build a coaching business, and launched the first Crush Your Money group coaching class online in March 2020.

Monetizing what comes naturally to you

The lesson Joy learned — focus on monetizing something that comes naturally to you — is similar to billionaire Mark Cuban's go-to career advice.

As a child, Cuban loved basketball. Even without NBA-level athletics, he still could've carved out a career working somewhere in the basketball industry.

But after analyzing what he was genuinely good at, instead of what he loved, he realized that he should probably pursue a career in technology, he said in a 2018 Amazon Insights for Entrepreneurs video.

"The things I ended up being really good at were the things I found myself putting effort into," said Cuban. "When you look at where you put in your time, where you put in your effort, that tends to be the things that you are good at."

Cuban sold his first tech startup, a software company called Microsolutions, for $6 million in 1990. His second company, an audio streaming service called Broadcast.com, sold to Yahoo for $5.7 billion nine years later.

"A lot of people talk about passion, but that's really not what you need to focus on," said Cuban, who's now a co-owner of the NBA's Dallas Mavericks. "You really need to evaluate and say, 'OK, where am I putting in my time?'"

Want to make extra money outside of your day job? Sign up for CNBC's new online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories. Register today and save 50% with discount code EARLYBIRD.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.