It would be possible to emphasize the bad things that have happened in the past, and continually to fill one’s mind with thoughts of what might have been. But what would be the use? We cannot change the past, no matter how much we might wish to do so. And to become preoccupied with negative thoughts can only feed negative emotions such as resentment and revenge. The outcome must necessarily be very far from pleasant. This is not the way to achieve positive progress and to live a happy and constructive life.
Remembering bad things with persistent ingratitude offers no healing for misfortune and can only bring unhappiness. Epicurus recommends a diametrically opposite approach that offers help and healing. This is the message of Vatican Saying 55 (quoted below): by cultivating gratitude not only can we feel better about the past but we can actually cure misfortunes.
Curing misfortunes cannot mean reversing what has happened, since (as the saying indicates) that is impossible. It may mean that in the way we think about misfortunes we can as far as possible turn them to good and neutralize their bad effects; perhaps most pertinently, we can cure the negative effects that misfortunes have on our own way of thinking, and thereby ensure that misfortunes (which are inevitable) will not cause us irremediable distress and undermine our opportunities for happiness.
The same idea is expressed in a quotation which Plutarch includes in his work Against Epicurean Happiness (§18; Usener 436):
Remembering good things that have happened in the past is of the greatest importance for a pleasant life.
Thoughts for the Day, September 12: ‘The cure for misfortunes lies in gratitude for what has been lost and the realization that it is impossible to undo what has been done.’ (Vatican Sayings 55.)