This story is part of CNBC Make It's Six-Figure Side Hustle series, where people with lucrative side hustles break down the routines and habits they've used to make money on top of their full-time jobs. Got a story to tell? Let us know! Email us at AskMakeIt@cnbc.com.

Alexandra Rutkay rejoiced when her son became 18 months old. She could finally ditch her diaper bag and pull her neglected designer purses back out of her closet.

But as her family of three headed to dinner in New York in August 2021, she realized she couldn't shove wet wipes, snacks and an emergency pull-up into her sleek Saint Laurent crossbody. "I was so mad about it," says Rutkay, now 41. "I thought, 'There has to be something better for us moms. Everything is so frumpy and stupid and nothing is cute.'"

Rutkay is a full-time makeup artist who sometimes works up to 16-hour days on movie and television sets. Her side hustle, Citymouse, sells cross-body diaper bags with changeable straps. It launched in June 2022, and has brought in nearly $600,000 in revenue so far this year, according to documents reviewed by CNBC Make It.

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At first, Rutkay didn't tell her husband David about Citymouse, because her previous side hustle attempts weren't always successful, she says. Now, he is the company's only full-time employee, taking a pay cut to run its operations, logistics and customer service. Collectively, the pair plan to pay themselves about $150,000 from the startup this year, Rutkay notes.

Rutkay herself spends 40 hours per week working on Citymouse — on top of her day job — because once she starts a project, she can't let it go, she says. She's also more driven to build wealth, and eventually pass it onto her son, after a health scare: In 2020, she was diagnosed with a soft tissue sarcoma, a rare form of cancer, in her right thigh.

After surgery and chemotherapy, Rutkay was deemed cancer-free in 2021. "Not to be morbid, but if I had died before [my son] even got to know me, what would I have left him?" she says.

Here, Rutkay discusses how other people can follow her path, what she'd change if she could start over again and the most overrated business advice she's ever heard.

CNBC Make It: Is your side hustle replicable?  

Rutkay: Yes and no. I think creating a business that fills a need is the best possible way to grow and sustain a brand — a lot of people can do that. But it takes a lot of luck for your project to fill that specific need at the right time.

Consistency [of effort] is everything. I treated my past side hustles like hobbies, not businesses.  

Knowing when to pivot is also important. When I started Citymouse, I took a six-week course with a spiritual life coach and realized I felt very badly about my previously failed [side hustles]. She reminded me that knowing when to walk away is actually a superpower.

How do you determine when it's time to walk away from a side hustle that isn't working?

I think the world just tells you. If you're doing everything in your power to get your product or service out there through marketing, and people aren't responding to it, that's obviously a sign.

After I had a baby in February 2020, I had this idea to make a pizza-themed baby milestone blanket. I invested about $6,000 from my previous Etsy shop, found a manufacturer overseas and launched it through Amazon FBA's program.

That was my genius idea, and it was the biggest financial mistake I've ever made. I didn't spend money on advertising because I thought if I made a listing with strong SEO, people would magically find it. That was absolutely not the case. Then, Amazon started charging me all these other fees.

I started panicking and liquidated the company four months after it started.

If you could go back in time and change the way you launched Citymouse, what would you have done to help it succeed faster?

I always thought I was my ideal customer, so I originally positioned Citymouse like a luxury brand for other city moms. It turns out, suburban moms who want to leave their giant diaper bags in the trunk of their cars also need a smaller bag for day-to-day errands like grocery shopping and Target runs.

There is a big difference between this and anything else I've ever made. I'm so passionate about Citymouse, and that's because there's an emotional element to it — for me and for other moms. I think finding that emotional element and figuring out how to make people feel something with your product, that's how you hit gold in business.

What's the most overrated business advice you've ever heard?

I know people say this out of the goodness of their hearts, but I cannot stand when you say you have an idea, and someone says, "You should go on Shark Tank."

You do not need outside investments to grow a profitable business. In fact, I would go as far to say, "Don't do that." I think there's such beauty and underrated power in bootstrapping your company. You have full say on everything, you're making the decisions yourself. It's empowering.

That doesn't mean I'm opposed to [selling equity in your company] forever. In my wildest dreams, in the next five to seven years, I sell Citymouse and I start doing else. I know people say not to start a company with the intention to sell, but I know myself. I'm a serial starter. I'll probably want to pivot to a different line, a different product, a different industry, eventually.

But it would be nice to exit on top.

This interview has been edited and condensed for clarity.

Correction: This story has been updated to reflect that Rutkay didn't initially tell her husband about CItymouse because her previous side hustle attempts hadn't always succeeded.

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