What's the Hardest Fold You've Ever Made

Every poker player remembers at least one painful fold that they still think about. Mine was laying down a strong hand because something just didn't feel right—and later I found out it was the correct decision.
What's the hardest fold you've ever made, and did you ever find out if it was the right one?

In every tournament I play, there's always one hand I regret folding and another one I know was the right fold. The problem is that, most of the time, the wrong decision ends up costing you your place in the tournament.

It always sucks, when you thought, you were betting for value, but then you get raised on the river and need to reevaluate and decide to fold.
Or when you flopped the nuts, but the board runout was really bad. Like you had on , but the turn and river runs out , and now your opponent bombs the pot. But I dont collect bad memories from poker of any kind, so really I cant say, which was the "most painfull fold". I play a lot, so I guess, you can say, I need to make "painfull folds" basically every single session.