This is a quote spoken by Buddha (and there have been many variations of it).
If you tell yourself that something is too hard, you will begin to believe it and prevent yourself from fully applying yourself and giving all you can give. If instead you think positively, you will believe that you can achieve your goal which will give you the strength to fight the battle.
If you put garbage in, you will get garbage out. Feed your mind with things that will help you grow and motivate you to become a better person. If you can conquer your thoughts you can conquer your battles.