Doubt is something that can kill dreams far more effectively than failure.
When you doubt you start to believe that your dreams are impossible to achieve, and this state of mind is both negative and damaging. When you fail this may motivate you to try harder so that you succeed the next time you attempt to reach your dreams.
If you doubt yourself and your abilities then the battle is already lost and your dreams will always be out of reach.
Doubt can be like an invasive parasite, slowly eating away at your confidence and eroding your determination. Instead of doubting yourself look at all the impossible tasks that men and women have achieved over the years and use this as motivation to push yourself even further in order to achieve your dreams and reach your goals.