A driver buckles up before pulling away, not a second before a crash. Trading works the same way. While there’s no trade, you think calmly. Once the trade is open and the price is jumping, you have to decide under fear or excitement, and your head works worse.
Example. You bought without a plan, and the price went down 3%. “I’ll wait, it’ll come back.” Down 6%. “It’s too late to sell now.” Down 12%. Each decision seemed sensible on its own. With a plan it’s simpler: you decided in advance to get out at −3%, you got out and lost a little.