Stripe, the payments company founded eight years ago by two Irish brothers, is now worth $9.2 billion.

Patrick Collison, the 29-year-old CEO of Stripe, says he still wakes up 20 emails in his inbox telling him all the things Stripe is doing wrong.

On an episode of NPR's podcast "How I Built This" with Guy Raz, Collison shared what it's really like running a unicorn startup.

Patrick Collison, 29, and his brother John, 27, famously turned seven lines of code into a startup called Stripe, which has been valued in fundraising at $9.2 billion.

According to Collison, who was recently named to Business Insider's list of rising stars in tech, running a successful startup doesn't get any easier.

On an episode of NPR's excellent podcast "How I Built This" with Guy Raz, the Collison brothers shared the story of how they founded and sold their first company before they turned 20 and went on to build Stripe, a software company that uses just a few lines of code to power the payment systems of companies like Target, Lyft, and Kickstarter.

When Raz asked when did Collison sit back and realize Stripe was a huge success, Collison said that "there's never really been time for that."

"There's nothing like a young company to every morning remind you that there's so much left to be done, so much that's not yet working the way it should be," Collison said on the podcast. "I mean, it's really quite visceral. You wake up in the morning, and there are 20 emails in your inbox that are sort of somehow all related to things that you're doing badly or wrongly. There is never a moment when it feels successful."

Collison said he often thinks about a famous quote from Greg LeMond, a professional cyclist who won two world championships and three Tours de France: "It never gets easier — you just get faster."

Collison, who said he used to cycle a lot, added:

"There's a lot of sort of painful truth to that where, as you cycle more, as you practice more, as you get fitter, as you get faster, as your form gets better, sure, you start cycling faster, your times get better. But the experience of being on the bike never gets easier. The pain that you feel on the first bike ride, that's the same pain that you're going to feel on your 500th bike ride. You'll just be going much faster on the 500th bike ride.

"And it kind of feels like that in a startup, where every day now the problems and challenges and, you know, visceral pain is just as acute as when we were starting out. The problem is just in a different form. It's this kind of relentless process of trying to shift what it is that exists and what we've collectively managed to create so far into what we all set out to create in the first place. And we still have quite a ways to go there."

In 2018, Stripe is going after larger customers such as Allianz, Booking.com, and Zillow. The company earlier this year also purchased a point-of-sale software developer backed by the former Alphabet chairman Eric Schmidt, something that will give Stripe a formal product for physical stores. Now, Stripe can tell its new breed of large customers that it can handle all kinds of payments.

You can listen to the Collison brothers episode of "How I Built This" on NPR.