Sallie Krawcheck has essentially lived two different lives, at least professionally.

For decades, Krawcheck worked in corporate America, rising through the ranks on Wall Street, holding prestigious positions including CFO at Citi and CEO of Merrill Lynch. But now, she's the CEO at a start-up. In 2014, Krawcheck founded Ellevest, a digital investment platform for women, and dove into the start-up life.

In terms of the environment change, Krawcheck was mentally prepared for some things; she knew not to expect all the amenities of a cushy, corporate job. But no one can really prepare you for the intense stress that comes along with running a start-up, she says, and the stress that comes with losing money on a daily basis.

"Because it's a start-up, for some period of time — perhaps years — you're going to be losing money," Krawcheck tells CNBC Make It. "And every day you come into the office, you're losing money."

In fact, Krawcheck points out that one of the biggest differences between leading an already profitable company and running a start-up is that at a start-up, you're raising the money yourself, which makes the stress much more significant. While all businesses have to deal with bad days, at a start-up, Krawcheck explains, the lows feel lower and more personal.

"If you have a bad day when you work for a profitable company, 'Yeah, we had a bad day we didn't push the business forward as much today. But look how much money we just made,'" Krawcheck explains. "When you're at a start-up, if you have a bad day you've lost money, and then you haven't done what you need to do in order to raise the next round [of funding]. So you sort of have a double bad day."

The weight of that daily stress and how to handle it is not talked about as much as it probably should be, according to Krawcheck.

Despite the pressure, Ellevest is flourishing. So far, it's raised $44.6 million in funding (tennis star Venus Williams is an investor) and has an $83.3 million valuation. Krawcheck and the company's mission is to close the gender investing gap by calculating financial goal targets to meet needs unique to women, like a larger retirement amount for a longer lifespan (women tend to live longer). Ellevest bases its contribution suggestions on the client's financial profile, while taking into account a gender-specific salary curve (for example, women tend to make less than men, and their salaries tend to peak at younger ages then men).

Of course, founders of start-ups are famously known for their intense passion, drive and determination for making their businesses successful. Krawcheck, too, says she has thrown everything into Ellevest and has previously even said it's harder to be an entrepreneur than to run Merrill Lynch. That type of hard work and personal investment (she says her team logged over 200 hours of research and interviews with women when creating Ellevest) tends to make the great days extra spectacular.

"The great days are great," Krawcheck admits. "The great days are greater than the great days in corporate America. But the bad days are worse. So you've got a higher beta (a measure of volatility or systematic risk)."

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