The problem isn't knowing, it's doing
Almost every trader knows what they should do. The worst losses come when emotions take the place of the plan: an early entry, a moved stop, one more trade to win it back.
The most common traps
Fear of missing out (FOMO): you enter late, on a move that has already started, with a stop that's too wide. Loss aversion: you hold losers hoping they'll come back and close winners too early.
Overconfidence after a winning streak: you raise the size or loosen the rules just as the market changes. Revenge trading after a loss: you want the money back right away and multiply your mistakes.
Measuring emotions
Note your emotional state before each trade, in a single word: calm, tired, impatient, euphoric. After a few dozen trades compare the average result in R for each state: the numbers will tell you when not to trade.
Rules that protect you from yourself
A daily stop after a fixed number of losses, a maximum risk per trade, a mandatory checklist before entering and a break after every closed trade. Rules work if you write them when the market is closed and enforce them when it's open.