Retrospectres are apparitions from your past that provoke intense, often debilitating emotional responses. They may take the form of a person you run into on the street whom you used to know. They may take the form of a scent or a taste that brings a flood of recollection.
They haunt places you have been, thought and acted and the act of returning to the scene of the crime, as it were, is enough to attract them to you like moths to a flame.
Retrospectres can be distinguished from memories by their autonomy: we are unable to control them. They come unbidden, wreak havoc with your mind and then disappear once they've had their fun. We have no say in the matter.
In theory, retrospectres could be positive entities. I myself have never experienced this. Encountering one usually feels like picking a scab; reopening an old wound.
How to deal with them:
Stay away from places you've had bad experiences! Failing that: take friends for moral support or try and think happy thoughts. Remember: everyone experiences a retrospectre haunting. It is helpful to consider the Buddhist principle of the present moment being the only real moment, in as much as time is ever real at all.