What is the hardest folding decision you have ever made in poker?

The hardest folds are usually when you have a very strong hand, but deep down you know your opponent only plays that way with something better. Folding a set, top two pair, or even an overpair in a massive pot takes real discipline — especially when the pressure is high. What do you think guys?

Any QQ fold to an higher set takes too much out of me. Because it is just luck playing fool.