The fact is that in order to do any thing in this world worth doing, we must not stand shivering on the bank thinking of the cold and the danger, but jump in and scramble through as well as we can.

Sydney Smith (1771-1845) English clergyman, essayist, wit
Lecture (1804-1806), Moral Philosophy, No. 9 “On the Conduct of the Understanding,” Royal Institution, London
    (Source)

Collected in Elementary Sketches of Moral Philosophy (1849).