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39-year-old retired millionaire: This part of retirement is 'almost more challenging than going to work'

In 2016, Chris Reining walked away from corporate America at age 37 with more than $1 million in the bank by living a modest lifestyle, saving more than half his income and putting his money to work. For nearly three years, he's been living comfortably off of his investments in Madison, Wisconsin.

Reining, now 39, doesn't miss working, he tells CNBC Make It: "My life just slowed way down. I'm a lot more relaxed and comfortable, and now, I can't imagine ever going back to a 9-to-5 job again."

That said, retiring early — and retirement in general — can be a big adjustment. The hardest part "might be figuring out what's important to you," he says. "When you're looking to retire, either early or even later in life, then you have to figure out what you're going to do with your life — and I think that's almost more challenging than going to work because then you actually have to figure out how you're going to live your life."

After all, "so much of your identity is tied up in what you do for your profession," he says. Think about when you're at any sort of social event or meeting someone for the first time: "One of the first things that someone asks you is, 'What do you do?' Once you get rid of what you do, you have to really dig deeper and figure out what is important to you."

The answer is going to look different for everyone, says Reining. For him, "it's been a process," and "really opened up an opportunity to grow. I've been spending most of my time reading and learning and studying. I've been spending time managing my investments, writing for my website and just thinking."

He's also using his extra time to take care of himself and invest in his health: "I work out way more now than I used to. I have a regular yoga practice. I got into meditation."

"When you're looking to retire, either early or even later in life, then you have to figure out what you're going to do with your life." -Chris Reining, early retiree

Another challenging aspect to early retirement is more technical: Transitioning your focus from accumulating money to withdrawing it.

"That's a shift that's not so easy," says Reining. "Because if you spent 10, 20, 30 or even 40 years saving and investing for your future, and now you have to make that switch in your head and start withdrawing that money, it's like going in the opposite direction."

Reining's withdrawal strategy is simple: He transfers the amount of money he projects he'll need for the year from his investment account to his checking account on January 1. "It's one large, lottery-sized paycheck," he says. "And that amount gets me through the year."

Don't miss: Here's how much money to save if you want to retire 'rich,' according to wealth manager David Bach

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