The sunk cost fallacy
Researchers have tested this many times, in different places, over many years, and they keep finding the same result. This one is safe to trust.
A group of researchers ran a simple study with theatre tickets. Some people paid full price for a season subscription. Others, chosen at random, were given a discount, the same seats for less money. Then the researchers watched who actually showed up over the season. The people who had paid full price attended more plays. The people who got the discount skipped more of them. The money was already spent in both cases, gone either way, and the tickets were identical. But the ones who had paid more felt they had more to waste, so they dragged themselves out on cold nights to use what they had bought. The size of the sum already spent, money that could not come back no matter what they did, was steering their behaviour weeks later.
You keep going on things because of what you already put in, not because of what they will return from here. The time, money, and effort already spent are gone whether you continue or stop, but they pull on you anyway, and the bigger the sum, the harder the pull. That is why the project you have poured a year into is the one you cannot walk away from, even when a smaller, fresher idea is clearly better. The only question that matters is the one the sunk cost hides: starting fresh today, knowing nothing about what it cost, would you begin this? Part of why stopping hurts at all is loss aversion.
Sunk cost says what you have already put in is gone and should not steer what you do next. Chesterton says what was put there before holds a reason you have not recovered yet. They are not about the same past. One is money, time and effort already spent, which cannot be recovered and should carry no weight. The other is the information encoded in an earlier decision, which can be recovered and should be, before you act. Ask "would I start this today" about the cost, and "why was this here" about the rule.
Source: Arkes and Blumer, 1985, Organizational Behavior and Human Decision Processes.
Annie Duke, 2022
A working method for ending things at the right time, from someone who had to learn it professionally.
Draw your own card. It does not take long, and it rewards taking your time.