Your memory is rewriting history
Left to memory, you will remember your wins as skill and your losses as bad luck, quietly editing your own track record into something flattering and useless. A journal exists to stop that. By recording what you actually thought before you knew the result, it replaces a story you tell yourself with data you can learn from.
Log the decision, not just the result
The most common journalling mistake is recording only outcomes — won, lost, profit, loss. That tells you almost nothing, because in a noisy market a good decision can lose and a bad one can win. What matters is the reasoning: the price you took, the probability you estimated, why you thought there was value, and how you sized it. The result is the least informative column.
What to capture for each position
Keep it light enough that you will actually do it: the date and the price, your estimated probability versus the implied one, the stake as a percentage of capital, a one-line reason for the entry, and where the closing price ended up. That last point lets you track closing line value — a forward-looking grade of decision quality that shows whether you are sharp long before the profits confirm it.
The review is where the value is
A journal you never reread is just typing. The payoff comes from periodic, calm review: are the bets you graded as high-value actually beating the close? Are your losses clustered in one market or one mood? Patterns invisible in the moment become obvious across a hundred logged entries — and that is how a record quietly turns you into a better investor.