The sunk cost fallacy is our tendency to stick with a choice because we've already invested time, money or emotion into it, even when continuing no longer serves us. The cost that's already been spent is called "sunk" because nothing we do brings it back, and yet we let it dictate what we do next.
Picture sitting through a film that bores you, but staying to the end "because I paid for the ticket." Or carrying on with a degree, a job or a relationship that exhausts you, reasoning that "I've put in so many years, it'd be a shame to lose them." In truth those years are already gone; the only thing you're choosing now is whether to add more.
Psychologically, the fallacy feeds on the fear of loss and on our need to avoid feeling that our effort was "for nothing." Admitting that something didn't turn out as we hoped can feel like failure, so we press on to protect our self-image. In doing so, we punish our future for the sake of a past that cannot change.
The antidote isn't to quit easily, but to ask: "if I were starting from scratch today, would I say yes again?" Often the difficulty of leaving hides deeper patterns around worth and guilt. If you recognise yourself here, working on it with a professional can help; see our services for a first step.