clipped from: www.sciencedaily.com   
Painful, emotional memories that people would most like to forget may be the toughest to leave behind, especially when memories are created through visual cues,
found that even "mild" emotional events, like getting a bad grade on a test or a negative comment from a coworker, can be hard to forget.

When people are trying to intentionally forget information, they need to mentally segregate that information and then block off the information they don't want to retrieve, Payne said.

Emotion undermines both of those steps. "You make a lot of connections between emotional events and other parts of your life, so it might be difficult to isolate them. As far as blocking retrieval of an unwanted event, emotion makes events very salient and therefore highly accessible," Payne said.

found that their subjects could not intentionally forget emotional events as easily as mundane ones. They also found that both pleasant and unpleasant emotional memories were resistant to intentional forgetting.