"I have learned to use the word impossible with the greatest caution." — Wernher von Braun
Impossible is a state of mind; what is impossible today may not be so tomorrow. What is impossible for us may not be so for others. This means that nothing is impossible if we know how to do it. To think otherwise will prevent us from finding a solution. Take flying for example. In the past, man could not fly. To make such a suggestion then would have made a person look stupid. But because some people refused to believe that flying was impossible, it is possible today. In fact, flying is so common that we take it for granted. The impossible has become the possible. When new changes or ideas become the accepted norms for us, we have greater options at our disposal. Today, we can travel by land, sea or air. In the future, space travel might become common for the masses.
It's easier to say something is impossible, or at least extremely unlikely. Everyone has periods of doubts. Everyone considers giving up sometimes, but then you just have to remember why you tried so hard in the first place.
Nothing is impossible. If you never tried it then you would never know if it worked. Every failure teaches you something if you are willing to learn from your mistakes. Those saying it can not be done should not interrupt those trying it. Artificial roadblocks are wasteful and counterproductive. Keep trying. Keep learning.
Your self-belief as a leader is infectious as well. What do you believe about yourself? What do you believe about your ability? What is possible and what is impossible? Your willingness to try the impossible will inspire your team push the limits as well.