If your investment thesis for a particular stock no longer holds true, do you sell immediately and take a loss?

Let's say you're holding a stock because you entered it for X and Y reasons. You see it slide for months and you revisit your initial thesis for entering the trade. X and Y no longer hold true, or perhaps you discovered a Z reason that really invalidates your initial thesis..

You decide you don't believe in the stock anymore. Do you sell at a loss immediately? Ladder out of the position as it recovers?

I know selling immediately comes across as "buy high sell low" behavior, but what do you guys usually do in these situations?

If you hold a position you no longer believe in, hoping to unload at a better price, doesn't that also go against the sunk cost fallacy ("would you buy the stock at the current price?")