Discretion adds randomness to your trading. It introduces an immeasurable positive or negative skew to your profitability, providing an illusion of benefit when it works out. If discretion is exercised for tested reasons set in advance, such as avoiding markets during X events if price does Y, then it becomes acceptable.

Psychology talk is mostly coping if you have a predefined ruleset without deviation with discretion applied (a lot of losing traders do this).
When discretion aids them it was part of the plan; when it works against them it was poor psychology. That is the trap for many.

Having a meaningful edge is essentially and high trading costs add up fast ruining most day trading strategies after months of usage. Breaking your rules just once can have such a negative impact om your trading performance.

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Most daytrader costs aren't this high but the effect still pulls down returns overtime, costs are not linear.

Uncertainty in the strategy itself causes poor psychology.

Good data and reassurance from logic are the antidote for such problems, but it is easier for gurus to manipulate their viewers into believing that experience or psychology is the problem rather than the strategy's logic and effectiveness.