What is the sunk cost fallacy?

Staying in a losing position because of what you already put in. That stake is gone either way, only the future counts.

"I'm down too much to cash out now" is the fallacy in one sentence. The market doesn't know what you paid and doesn't care; your shares are worth today's price whether you sell or sulk. The only real question is: would you buy this exact position, at this price, right now?

If the answer is no, the cash-out button exists precisely for you. Every bagholder in history is made of sunk cost, holding not because the odds say hold, but because selling would make the loss feel official. It already is.