Time machines. Who doesn't dream of having the chance to go back and right a wrong, to take a do over, to say yes (or no) when we should have. I'm not saying to live with regret--there's no point in post-mortem'ing your life (is that a word?)--but the luxury of time and reflection does lead to wisdom.

For me, I would tell my 25-year-old self not to raise money from certain specific people I now know who are the worst forms of incapable and arrogant humanity. I'm kidding (But not. Call me. I'll tell you more.) Seriously though, I would take the time to build authentic relationships.

At 25 I had no kids, no family obligations--I had all the time in the world (besides working my butt off) to socialize and get to know people better. To forge true friendships. To get solid advice from my peers and mentors. And those relationships matter even more 15 years later. Those are the people I turn to for advice I can count on; for capital when I'm starting a new company; for introductions that will make a difference.

I asked several entrepreneurs, all on the horizon of 40, if they could turn back the clock, what would they tell their younger selves?

Betty Liu, Bloomberg TV anchor and founder/CEO of Radiate

"Stop being so hard on myself. We are our worst critics and I spent too much time in my 20s telling myself I couldn't do this or that because other people were smarter or better than me. And then it dawned on me as I got older that everyone else succeeding was no smarter or better; some very successful people made lots of the same dumb mistakes I made, too.

And so I stopped the negative self-talk and just went for things, knowing I might fail miserably at some but also succeed exceedingly well at others. That's life!"

My Take: I would tell also myself to stop taking yourself so seriously and to lose the ego. Lose the insecurities and stop competing with others for no reason--I would tell myself that I'm not a big deal no matter what I accomplish. I now realize that it doesn't matter what everyone else is doing. I have my own story to tell and it doesn't matter who builds their first million-dollar company faster.

Charlie Kroll, cofounder Ellevest

"Seek out diversity. You may not realize it now, but when everyone around you (employees, investors, board members) comes from a similar background, you'll miss out on important ideas. You say constructive debate is important to you, but you're not doing enough to foster it. Go seek out people from different socioeconomic and educational backgrounds, races, genders, and sexual orientations, and show that it's safe to disagree and challenge status quo. You'll make better decisions as a result."

My Take: Diversity is vital to the success of a company--I need people who can notice my blind spots; that can tap into the mind of every consumer; that are willing to and feel empowered to challenge me and my opinions. It's what makes us all better.

Kai Bond, Investor, Comcast Ventures and Catalyst Fund

"Go and start a company now. Don't wait. Don't go to another startup to learn. Don't go to a big company to refine your skills. There is only one way to be a truly great founder and that is to completely immerse yourself in every aspect of entrepreneurship. It took me more than 10 years to become a truly good founder and I wish I would have started building more of those core competencies earlier in my career." 

My Take: I have found there are truly great founders (like Kai), but there are even more founders who cannot scale. And remember, founders can get fired in an instant if your board isn't your number one fan.

Words to remember...

It takes gumption to better yourself, to ask for feedback, and to have the benefit of real-world experience. I wish I had started with an executive coach earlier in my career. I truly believe anyone, at any stage in their life, can benefit from it. They're almost like your own personal time machine; they know what you know, but have this omniscient power to make your future self better.