When you're in your 20s, you can't know everything life has in store for you. But while you might not know the name of your future spouse or what twists your career might take, there is still one certainty: There will be struggles. 

Even the happiest lives are studded with disappointment, failure, inner turmoil, and outer obstacles. There's nothing you can do about that, so your best bet is to get good at weathering setbacks early in life.

How do you do that? Psychologist Nick Wignall recently offered a handful of great suggestions in the form of a letter to his younger self on Medium

"We don't have much control over when and how tragedy strikes in our lives -- and we don't have much control over the genes we inherited or the family we were raised in -- but we do have control over our habits, including our habits of mind," he writes before offering this advice on how to start building mental toughness early. 

1. Cultivate strong beliefs weakly held 

Jeff Bezos, as well as other incredibly smart investors and researchers, swear that "strong beliefs weakly held" are key to maximizing your intelligence. Wignall claims this same quality of intellectual humility and openness to changing your opinions on the basis of new evidence also makes you more resilient. 

"Don't be afraid to cultivate and defend your beliefs with strength and passion. Have the courage to take a stand and argue for what you believe in," he instructs his younger self. "But have the humility to know you don't know everything. Life goes on, circumstances change, new information shows up -- whether you want it to or not. Be willing to embrace new facts and adjust your priors. You can't force reality to fit your beliefs." 

2. Practice validating emotions 

You could live to be 100 and you'd never learn to repress your emotions and still be mentally healthy. Running away from your feelings never works. Instead, get good early in life at acknowledging them without letting them control you. 

"You know what separates people who are truly great from those who are pretty good?" Wignall asks. "It's the people who can manage their emotions effectively." Developing that capacity comes down to practicing validating how you (and others) feel. 

"Validating emotions means that you're willing to approach them and acknowledge them instead of suppressing them or distracting yourself from them. It means acknowledging that no matter how painful or unpleasant, emotions are not bad or dangerous. They're not viruses to be eradicated. In fact, they're often valuable messengers containing useful information," Wignall explains. "Learn to be accepting of your emotions and they will work for you instead of against you." 

3. Meditate 

Yes, you've probably heard you should meditate about 1,000 times before, but there's good reason for that. "It's the best way BY FAR to strengthen your ability to control your attention. And the ability to control your attention -- to hold your focus on one thing for long stretches of time, or to disengage your mind from patterns of thought that are unproductive -- is an absolute superpower," Wignall insists. 

If you don't believe him, there's a ton of science to back up his claim that meditation is incredible strength training for the brain

Do these three things and you'll enter your fourth decade of life with a stronger mindset to handle whatever life throws at you. And, hey, even if your 20s have long passed you by, Wignall's post is a great reminder that we would all benefit from cultivating these mental habits. You're never too old to get mentally stronger.