Brooding over the past and its horrors is a sign of insanity, a disease that makes it impossible to focus on the here and now. People with clear goals have stored and forgotten past events that will never again come to light because they are so deeply ingrained in their minds. Past events are over; regret cannot recover them, melancholy cannot right the wrongs, and misery cannot bring the past to life. This is due to the nonexistence of the past.

         Avoid dwelling on regrettable memories or living in the shadow of what you have missed. Protect yourself against the spectral presence of the past. Do you believe you can bring the sun back to where it first rose, a child back to its mother’s womb, milk back to the cow, or tears back to the eyes? You put yourself in a very dreadful and terrible state of mind if you are always thinking about the past and what happened.
It is a waste of the present to read too much into the past. Former days are gone and done with, and you gain nothing by performing an autopsy on them or by turning back the wheels of history. When ALLAH mentioned the affairs of previous nations, he, the Exalted, said: “That was a nation who has passed away.” (Quran 2:134) Former days are gone and done with, and you benefit nothing by doing so.
  Living in the past is similar to trying to saw through sawdust. In the past, people used to warn against digging up the dead. Tragically, we are unable to deal with the current and instead neglect our magnificent castles while crying out loud about crumbling structures. Together, every man and every jinn would undoubtedly fail in their attempt to change the past.

You should move forward as the rest of the world does in order to get ready for a new season.

Source: Don’t be sad / Aaidh Al –Qarni; translated by Faisal ibn Muhammad Shafeeq, 2nd edition, Riyadh, 2005, International Islamic publishing house.