This is one of my favorites. When ever I feel demotivated or depressed, I read this quote several times. At such times, we should think of our previous achievements and motivate ourselves, thinking that when we could achieve that, we can achieve this too. Every one has achieved something, despite, difficulties and difficult circumstances or limitations. So we should remember all those things.
Moreover, we should also have confidence in ourselves, that we can achieve anything that we set our mind on. We should have faith in God as well as our efforts in achieving many things.