Sunk Cost Fallacy

The sunk cost fallacy is the tendency to keep investing time or money into something because of what has already been invested, rather than evaluating the decision fresh based on its current merits. Money already spent or already lost is gone regardless of what you do next, but it still exerts a psychological pull on the next decision.

In trading, it shows up as holding or adding to a losing position specifically because of how much has already been put into it ("I've already lost this much, might as well see it through") rather than asking, with a clear head, whether you would buy the position today at its current price if you didn't already own it.