Everyone experiences doubt, feelings of worthlessness, and failure at some point during their lives. Whether you're starting a business, working on a project, or simply discouraged by the day-to-day of your current career, looking to those who have experienced similar emotions and situations may help lift your spirits and help you stay optimistic throughout your current struggles.

"The biggest risk is not taking any risk ... In a world that's changing really quickly, the only strategy that is guaranteed to fail is not taking risks." - Mark Zuckerberg, CEO of Facebook.

"I treat my business as if it's about to be overshadowed by my competition. When you get too comfortable with your product and with your revenue, you lose the will to grow"- James Parsons, CEO of Blogpros.

"Life is too short to do mediocre work and it is definitely too short to build shitty things." - Stewart Butterfield, CEO of Slack.

"Startups are emotional roller-coasters: entrepreneurs can shoot from despair to euphoria multiple times a day. Some people hate that, some find it's the best thing in life." - Guillaume Decugis, CEO of Scoop.it

"Do it. You'll learn more in a year as a founder than you will in a lifetime of working for a big corporation. Joining a young startup is similar, but not quite as wild a ride as being a founder. In either case you're at the leading edge of the company defining what it will become. You have your fingers in a hundred different pies and get to work on the most interesting thing of all: something you've created." - Tom Preston-Werner, Founder of GitHub.

"When you find an idea that you just can't stop thinking about, that's probably a good one to pursue." -Josh James, co-founder and CEO Omniture, founder and CEO Domo.

"Go a little easier on yourself, and in so doing, be prepared to make and do things that might seem silly at first. Just keep moving: don't ruminate and stare at the wall. Don't just play with your phone: go out and produce something." - Merlin Mann, podcaster and founder of 43 Folders.

"Your biggest frustration can turn into your greatest idea."--Troy Osinoff, Entrepreneur

"All our dreams can come true, if we have the courage to pursue them." - Walt Disney.

"You have to be a brat in order to carve out your parameters, and you have to be a monster to anyone who gets in your way. But sometimes it's difficult to know when that's necessary and when you're just being a baby, throwing your rattle from the cage."--Paul Thomas Anderson, director.

"Writing your own blog platform is like roasting your own coffee: it's impractical and you probably shouldn't do it, but for people who really, truly care about it, it's worthwhile to them for their own personal priorities that sound crazy to everyone else. Well, I write my own blog platform and I roast my own coffee." - Marco Arment, founder of Instapaper and Tumblr.

"You should make something. You should bring something into the world that wasn't in the world before. It doesn't matter what it is. It doesn't matter if it's a table or a film or gardening-everyone should create. You should do something, then sit back and say, 'I did that." - Ricky Gervais.

"Confidence doesn't come from knowing you're right--it comes from being okay with failing." - Mike Monteiro, Mule Design.

"Start-up success is not a consequence of good genes or being in the right place at the right time. Success can be engineered by following the right process, which means it can be learned, which means it can be taught." - Eric Ries, author of The Lean Startup.

"Overcoming obstacles in business is directly related to overcoming obstacles in your life and applying absolute integrity passion and a sense of humor is everything in succeeding in a tough world."--Clair Parr, Co-Founder of Live In The Vineyard

"The more you praise and celebrate your life, the more there is in life to celebrate." - Oprah Winfrey.

"What I'm suggesting is, stand for yourself, be for something and the hell with it. Because the hand-wringers and the editorialists and the sigh-and-pontificate crowd will be against you, whatever you do." - James Carville.

"Successful entrepreneurs develop products that inspire their passion. They have to. It's that passion that gets them through the long, arduous, uncertain and frightening early days of a start-up." - Mitch Kapor.

"Don't be a lone wolf. Lean on the experience and smarts of your teammates, investors, and mentors to help solve the tough problems and take advantage of the opportunities." - Seth Bannon, founder and CEO of Amicus.

"Bad shit is coming. It always is in a startup. The odds of getting from launch to liquidity without some kind of disaster happening are one in a thousand. So don't get demoralized." -Paul Graham, co-founder of Y Combinator.

"I'm convinced that about half of what separates successful entrepreneurs from the non-successful ones is pure perseverance."--Steve Jobs, co-founder of Apple.

"Don't worry about failure; you only have to be right once." -Drew Houston, Dropbox Co-Founder and CEO.

"Don't limit yourself. Many people limit themselves to what they think they can do. You can go as far as your mind lets you. What you believe, remember, you can achieve." -Mary Kay Ash, Mary Kay Cosmetics Founder.

"What kind of founder do you want to be? Whatever you decide, stick with that definition with absolute and unapologetic confidence." - Jessica Mah, CEO of Indinero.

"Believe and set your sights on the fact that you can do it. It's certainly a goal any woman can have, just like any man." - Kathleen Kennedy, President of Lucasfilm.

"The faster you grow, the more problems you'll face. Start to see your struggles as a natural by-product of growth and face them with confidence. Whether you've run a company before or not, have a PHD or didn't graduate high school, none of this matters. Resilience is everything, and resilient leaders win." - Anthemos Georgiades, CEO of Zumper

"You could have a million ideas, but they're all worthless if you don't get them done." - Lauren Amante.

"Achievement is the reward given to those who stand in perpetual rebellion against discouragement, rejection, and failure." - Danny Done, CEO of Marketeering Group.

"The critical ingredient is getting off your butt and doing something. It's as simple as that. A lot of people have ideas, but there are few who decide to do something about them now. Not tomorrow. Not next week. But today. The true entrepreneur is a doer, not a dreamer." - Nolan Bushnell.

"Mistakes will not end your business. If you are nimble and willing to listen to constructive criticism you can excel by learning and evolving."--Meridith Valiando Rojas, co-founder and CEO of DigiTour Media.

"It's easy to get caught up in the day-to-day hard work of running a business and completely forget that while solving problems and building something you can be proud of, you also have an incredible opportunity to shape the future. Even better, you have the chance to meet amazing people and do amazing things along the way. It's important not to get lost among the trees and forget about the amazing, beautiful forest you're planting."--Colin Wright, Co-founder, Asymmetrical Press.

"Don't compare your chapter 1 to my chapter 15. Write your own success story."--John Rampton, founder of Due.

"It is through science that we understand the world around us, and by understanding the world around us, we not only contribute to ourselves, our family, to our communities, etc. - you also contribute to the basic development and evolution of humanity." - Rusty Schweickart, former astronaut and author.

"There's nothing wrong with staying small. You can do big things with a small team." - Jason Fried, 37signals (now Basecamp) founder.

"If you're not loyal to your team, you can get by for a while, but eventually you will need to rely on their loyalty to you, and it just won't be there." - Tim Schafer, founder of Double Fine Productions.

"It becomes important to choose something one is passionate about. If one enjoys the process, it becomes easier to get through the countless failures before the eventual success." - Ashutosh, Purplehed.

"I've always been a believer in continuing self-education. The more you learn, the more you earn... But more importantly, the more good stuff that goes into your brain, the better the stuff that comes out of your mouth."--Travis Wright, Chief Marketing Technologist, CCP Digital.

Everyone has a handful of memorable quotes they carry around with them to reference and consider when times are tough. Hopefully this selection added to the list and helped you find solace through your current situation. If you have some favorites of your own you think would fit in with this piece, please share them in the comments below - then get back to doing the work you're driven to do.