This has been my motto ever since a good friend sent me the book as a birthday present to a remote little Post Restante office in India. It was Jonathan Livingstone Seagull by Richard Bach.
At the time I was really struggling. First big trip alone outside Europe, Delhi belly, serious dehydration and generally feeling very overwhelmed. The book really made me think and I loved the message behind that quote.
Now, every time I feel a bit overwhelmed and those feelings of self doubt start taking over, I remind myself that I can do anything I set my mind to.
It's a philosophy that has served me well. Try it :)