Freud in 'Selected papers on hysteria' gave this explanation of how the affect related to psychological injury could resurface.

"It would seem at first rather strange that long forgotten experiences should exert so intensive an influence and that their recollections should not be subject to the decay into which all our memories sink. We will perhaps gain some understanding of these facts by the following examinations.

The blurring or loss of an affect of memory depends on a great many factors. In the first place, it is of great consequence whether there was an energetic reaction to the affectful experience. By reaction we here understand a whole series of voluntary or involuntary reflexes ranging from crying to an act of revenge through which, according to experience, affects are discharged. If the success of this reaction is of sufficient strength, it results in the disappearance of a great part of the affect. Language attests to this fact of daily observation in such expression as 'to give vent to ones feelings', to be 'relieved by weeping' etc. If the reaction is suppressed, the affect remains united with the memory. An insult retaliated, be it only in words, is differently recalled than one that had to be taken in silence... the reaction of an injured person to a trauma has really only then a perfect 'cathartic' effect if it is expressed in an adequate reaction like revenge. But more likely man finds a substitute for this action in speech through which help the affect can well-nigh be abreacted."

Freud sees the affect 'united with the memory' as an explanation for understanding why past hurts recur. Catharsis in the form of revenge or retaliation reduces the affect, making the memory less compelling. However forgiveness would bypass all this. Even when the affect is powerful and remains 'united with the memory' (i.e. the person is still angry when thinking about the offence) forgiveness is a process of dropping the offence, without catharsis necessarily occurring.