My dad used to quote this phrase all the time when I was growing up. He believed (and still does) that a positive approach to life could accomplish great things. Now, we aren't all equipped to do some things, but with perseverance and dedication, we can do a lot more than we realize.
And the second part of the quote is true, as well: if we choose to believe we can't do something before we have even tried, we never will.
Don't sell yourself short. Take a look at what talents and gifts God has given you. Pray for direction in what you hope to accomplish. Set a goal and chase after it with confidence in Him to bless your endeavors if it is His will. Stop saying "I can't," and you may be surprised what you are capable of accomplishing!
Journey on!