As I enjoy my 40th year on this planet, it's an exciting time to look back and think of the many, many mistakes I've made along the way. Ok, well, maybe I wouldn't describe that as exciting, but it is certainly valuable to look back and think about what I've learned in my first 40 years on this planet. As young leaders graduate from college and prepare to take on the world, here are 7 things I wish I knew when I was 21 years old:

1) It's much better to live in the present than in the past or future.

Living in the past will fill you with regret and depression, while living in the future will fill you with anxiety and unrealistic expectations. I know - I've been there. Plus, we really only have one choice, which is to live in the here-and-now, and to be present with ourselves and with others. So, limit your looking-back and looking-forward activities carefully, and for goodness sake, put your smartphone away.

2) Reading and writing sharpen the brain while TV news dulls it.

Just about everyone I know watches television. I watch television. But today, I avoid TV news at all costs- it's sensational, biased and depressing. I wish I hadn't spent so many hours in my twenties and thirties watching dumb TV. Instead, I wished I'd spent that time reading and writing. Now, I read and write more than ever before, and it's definitely made me a better thinker.

3) Listening is much more influential than talking.

This is so counterintuitive, I know: If we want to get something, we think we need to talk our way into it. But really, the opposite is true: If we want to get something, we need to listen our way into it. As Aaron Burr says in my favorite Broadway show Hamilton: "Talk less, smile more." Pay attention to what your colleagues, friends and family really want. It's more valuable to be interested than to be interesting.

4) Fear is normal. Feel the fear and go for it anyway.

Everyone's afraid. I'm so afraid of so many things - which is ironic, because I've often been called "fearless." But the important thing is to not let fear debilitate you. I was so afraid of rejection and failure before, it kept me from acting. Instead, feel the fear and go for it anyway. That's the definition of "courage", by the way.

5) Givers gain.

Another counterintuitive one: When I was younger, I didn't realize the way to abundance is to give of yourself, not to try to get as much as you can. Some of the most successful people I've met in the last 20 years are true givers: Adam Grant, Bob Burg, James Altucher, Jim McCann. The universe is truly amazing in rewarding those you give of themselves.

6) Taking care of your body is essential.

You've got to take care of yourself. No, not in a "me, first, get money" kind of way (See point 5). No, as my wife, business partner and fellow author Carrie Kerpen writes, you've got to take care of your body. This has definitely been the hardest lesson for me over the past 20 years - the food-loving, partying-hard, sleep-skipping guy that I am. But I wish I realized at 21 how much more energy you can actually have from proper nutrition, exercise, and sleep, and limited caffeine, alcohol and drugs.

7) Gratitude is the best drug on the planet.

Which bring me to my last point: the best drug in the world is gratitude. It's (usually) free and there are never any side-effects. Handwritten thank you cards, acts of kindness, gratitude lists - these are just a few ways to take your mood from bad to good, good to great, or great to ecstatic. Literally every time I'm down, I can take a hit of gratitude and feel better. You can too.

I love learning. In fact, I can't wait to write the "7 Things I Wish I Knew at 40" in 10 years or so. In the meantime, those are the 7 things I wish I knew at 21. What are yours?