he laughs best who laughs last
A person should go ahead with what he is doing and not worry when others laugh at him. When he succeeds he will enjoy laughing at them for being wrong more than they enjoyed laughing at him. — A proverb.
Everyone laughed at Mary when she was learning to ski. She kept falling down. Now she is the state champion. He laughs best who laughs last.
Source: A Dictionary of American Idioms