“You never know just what you can do until you try.”
“You never know how much you can do, until you try to do more than you can.”
“You don’t know what you can do. You won’t know until you try.”
“Until you try, you don't know what you can't do.”
“You don't know what you can get away with until you try.”
“You will never know what you are doing until and unless you have done it.”