“One never knows what they're capable of until they try.”
“No one knows what he or she is capable of until things are at their darkest.”
“So few people know what they're capable of.”
“You never know what's in a person's heart until they're tested, do you?”
“Fortune does favor the bold and you'll never know what you're capable of if you don't try.”
“You never know just what you can do until you try.”