He who hesitates is lost

A person who spends too much time deliberating about what to do loses the chance to act altogether.

One who cannot come to a decision will suffer for it, as in I couldn't make up my mind, and now the offer has expired—he who hesitates is lost. Although the idea is undoubtedly older, the present wording is a misquotation or an adaptation from Joseph Addison's play Cato (1712): “The woman that deliberates is lost.”