If you keep waffling, the opportunity will slip away.
Tips
grammar
This is a potential complement: 拿得定主意 (can make up your mind) ↔ 拿不定主意 (can't). The 定 means "fix / settle," literally "grab but can't settle on an idea." The affirmative 拿定主意 is also common: 他早就拿定了主意 (he made up his mind long ago).
