By brooding over the past and its tragedies, one exhibits a form of insanity - a kind of sickness that destroys resolve to live for the present moment. Those who have a firm purpose have filed away and forgotten occurrences of the past, which will never again see light, since they occupy such a dark place in the recesses of the mind.
Episodes of the past are finished with; sadness cannot retrieve them, melancholy cannot make things right, and depression will never bring the past back to life. This is because the past is non-existent.
Do not live in the nightmares of former times or under the shade of what you have missed. Save yourself from the ghostly apparition of the past. Do you think that you can return the sun to its place of rising, the baby to its mother's womb, milk to the udder, or tears to the eye? By constantly dwelling on the past and its happenings, you place yourself in a very frightful and tragic state of mind.
Reading too much into the past is a waste of the present. When Allah mentioned the affairs of the previous nations, He, the Exalted, said:
That was a nation who has passed away. (Qur'an 2. 134)
Former days are gone and done with, and you benefit nothing by carrying out an autopsy over them, by turning back the wheels of history.
The person who lives in the past is like someone who tries to saw sawdust. Of old, they used to say: "Do not remove the dead from their graves."
Our tragedy is that we are incapable of dealing with the present: neglecting our beautiful castles, we wail over dilapidated buildings. If every man and every jinn were to try jointly to bring back the past, they would most certainly fail. Everything on earth marches forward, preparing for a new season and so should you.
Source: Don't Be Sad - By Aaidh ibn Abdullah al-Qarni