Success is based on a number of factors. Intelligence, experience, talent, education all play their part...but often what separates success from failure is perseverance. Keep going and you still have a chance to succeed; quit, and all hope of success is lost.

So when the going gets tough and the odds seem long, how do you deal with the sense of doubt that starts to chip away at your motivation and willpower?

Start by accepting that doubt is a natural part of the process.

If you're even a semi-regular reader of mine, by now you know I love the reality series The Selection: Special Operations Experiment, which airs Thursday nights at 10 p.m. EST on History. (Or you can catch up on the show's site.) Thirty people with no military background undertake extreme physical and mental challenges; their instructors are combat veterans from various U.S. Special Operations units including Navy SEALs, Special Forces Green Berets, and Army Rangers.

Recently I had the honor of talking with one of the instructors, Navy SEAL Ray Care, about perseverance, developing the right mindset, and how the only limits we really have are self-imposed. Then I talked with another instructor, Army Ranger Tyler Grey, about adaptability, attitude, mental toughness, and how in life there is no finish line.

And because too much advice from incredibly smart people is never enough, this time I talked with another instructor, Navy SEAL Sean Haggerty, about pushing through doubt -- and how that ability is so important in business and in life.

You said during your BUD/S (SEAL training) you were constantly doubting yourself. How did you push through those moments?

Don't confuse doubting yourself with accepting failure.

Everyone questions whether they can actually accomplish something difficult while they're doing it. That's normal.

The best thing I did was to decide that I was going to go to the absolute extreme, even if I doubted myself. I basically told myself that no matter what, I wouldn't quit. I doubted myself a number of times, but then I put away and thought, "If I fail, I fail...but what I will never do is quit."

That attitude pushes you past a limit you think you have...but you really don't.

In one episode you led the group in doing 1,000 sit-ups. Anyone can do sit-ups, so a lack of skill couldn't be an excuse. That made the evolution a mental challenge: "Will I quit...or not."

You're right. Doing sit-ups is a simple thing. Trust me, there are not many things I'm really good at...and sit-ups definitely isn't one of them. That's a good example of taking people through an exercise they're really uncomfortable with. I don't know people who do sit-ups all day long.

In that episode the lesson is simple: intestinal fortitude. Really that's the whole premise of the show: Don't stop. Keep going. No matter how hard it seems, figure out a way and keep going.

I guarantee you that everyone lying out there in the sand doing sit-ups doubted themselves. We didn't tell them how many sit-ups they would have to do. How do you know if you can finish when you don't even know where the finish line is?

It really is OK to doubt yourself...but it is never OK to sell yourself short. Doubting doesn't mean you can't do it or won't do it -- doubting yourself is just a sign you need to figure out a way to keep going.

And by the way, we did more than 1,000 sit-ups.

How do you instill that attitude in other people?

One way is leading by example. Whether you're leading a small team, a large team, a team you've worked with for a long time...the one thing that translates to every situation is that selfless effort and leading by example always pays off.

Words only describe things. Actions get things done. When you're looking at your own performance or the performance of your team, always make sure your evaluation is performance based. Always look at what you or your team is doing, because actions speak louder than words.

Another key is to remember that even though you're in a leadership role, you still need mentors. What's great about the SEAL teams is that you work with some of the best people in the world.

And on The Selection, getting to work with Rangers, Green Berets, other SEALS...I learned so much from them because they have such different backgrounds.

Expanding self-imposed boundaries is a lifelong process. So is learning.

You're starting a brewery with some other veterans. How does perseverance translate to entrepreneurship?

I knew when I was getting out I wanted to be an entrepreneur. Startups involve the same kind of chaos: You have to come up with innovative solutions to complex problems, you're being pushed every day...and it's up to you to find the motivation and willpower to keep going.

No matter what you're trying to do, motivation has to come from inside you. You have to look inside yourself. You have to keep going no matter how hard it gets.

That's what successful people do. They may have doubts, but they keep going.