Launching your startup may well be the most exhilarating, rewarding, and terrifying experience of your lifetime. I remember that roller coaster well--there were a lot of sleepless nights in WordStream's early days, especially when I was looking for funding and the string of rejection letters was never ending. Looking back, of course, I can see that everything happened for a reason. Hindsight is 20/20 and all that; the real perspective usually comes long after you need it. But if you're stuck in the throes of startup angst, you need to keep perspective. It's tough when you feel like your entire existence hinges on the success or failure of this thing--this thing you're pouring your heart into, your baby, your life's work. Listening to the advice of experienced and successful entrepreneurs is a great way to stay motivated when startup life gets you down. Here are 13 incredibly successful millionaire entrepreneurs on why startups fail, with words of wisdom to help you avoid the same fate.

Accepting and Weathering Criticism

You're going to get some flack in your early days, no matter how epic your idea may be. There are always detractors. Here's what Steve Jobs, Jack Ma, and Larry Ellison have to say about dealing with the inevitable criticism. "Don't let the noise of others' opinions drown out your inner voice. And most important, have the courage to follow your heart and intuition." --Steve Jobs, Apple founder. "When I first started Alibaba, I was immediately met with strong opposition from family and friends. Looking back, I realized that the driving force for me then was not my confidence in the internet and the potential it held, but more of this: "No matter what one does, regardless of failure or success, the experience is a form of success in itself." You have got to keep trying, and if it doesn't work, you always can revert to what you were doing before." --Jack Ma, founder of Alibaba. "When you innovate, you've got to be prepared for everyone telling you you're nuts." --Larry Ellison, Oracle founder.

Building (and Keeping) Confidence

On top of the people who always seem to have something to say about your startup, you're going to have to deal with the most prolific critic of all: yourself. Here's what the founders of successful brands like Tesla Motors, Louis Vuitton, and Under Armour have to say about your optimal startup state of mind. "I think that's the single best piece of advice: Constantly think about how you could be doing things better and questioning yourself." --Elon Musk, founder of PayPal, Tesla Motors, and SpaceX. "Don't think you are unstoppable or foolproof. Don't think that the only way your business will work is through perfection. Don't aim for perfection. Aim for success." --Brazilian business magnate Eike Batista. "I think in business, you have to learn to be patient. Maybe I'm not very patient myself. But I think what I've learned the most is be able to wait for something and get it when it's the right time." --Bernard Arnault, founder of Louis Vuitton and Moet Hennessy. "There's an entrepreneur right now, scared to death, making excuses, saying, 'It's not the right time yet.' There's no such thing as a good time. I started an apparel-manufacturing business in the tech-boom years. I mean, come on. Get out of your garage and go take a chance, and start your business." --Kevin Plank, founder of Under Armour.

On Effort and Sweat Equity

Of course, startups are a lot of work. It's still surprising to learn just how hard some founders go at it in those early days. While you still have to be good to yourself and keep that positive state of mind, the most successful founders are often the most driven and willing to keep going long after others would have backed down. "I'll let myself obsess over things. To get GoPro started, I moved back in with my parents and went to work seven days a week, 20 hours a day. I wrote off my personal life to make headway on it." --Nicholas Woodman, GoPro founder. "The most dangerous poison is the feeling of achievement. The antidote is to every evening think what can be done better tomorrow." --Ingvar Kamprad, IKEA founder. "Failure is not the outcome--failure is not trying. Don't be afraid to fail." --Sarah Blakely, Spanx founder.

Staying True to Yourself and Your Vision

I think pretty much all startup founders reach a point at which they have to ask themselves, "Why in the hell am I doing this?" Don't lose sight of what it is that drove you to become an entrepreneur and start your business in the first place. Tech founders Tony Hsieh, Jeff Bezos, and Andrew Mason have some great advice on staying true to yourself and your vision. "Have fun. The game is a lot more enjoyable when you're trying to do more than just make money." --Tony Hsieh, Zappos founder. "If you never want to be criticized, for goodness' sake, don't do anything new." --Jeff Bezos, Amazon founder. "I've found that as long as you're fundamentally good--as long as you're not being bad to people--people give you a lot of room to be yourself, because being yourself is being honest. And that's what people want to see." --Andrew Mason, founder of Groupon.