The sunk cost fallacy is a powerful psychological bias. It's our tendency to continue with a project or endeavor because we've already invested time, money, or effort into it—even if it's clear that the project is failing.

"I've already put so much money into this car, I have to keep fixing it." "I've spent four years on this degree, I can't switch now."

The money, time, and effort you've spent are gone. They are "sunk." They cannot be recovered. The only rational question to ask is: "Knowing what I know now, and based on the future costs and benefits, what is the best decision to make *today*?"

Ignoring sunk costs is difficult. It feels like admitting failure or waste. But clinging to a bad decision because of past investments is just throwing good money (or time) after bad. The safest decision is always the one that is best for your future self, regardless of the baggage of the past.