Decisions to stop are almost impossible to make well in the moment, because the moment is always the worst one. A quit point is written in advance, when you have nothing at stake, and it is the difference between a bounded experiment and an open-ended one.
Useful quit points are specific and observable: a hard capital floor for the account, a drawdown beyond the historical maximum of the system, a date by which defined milestones must be met, a savings level below which you stop funding the attempt.
Deciding to stop is not failure, and the alternative is worse. Traders who never set a quit point tend to discover one at the point where there is nothing left to decide with.
Related: grit-vs-stubbornness, risk-of-ruin, scared-money, financial-stress