This is a tad morbid, but for people who have died without knowing Christ or had no interest in Him, we cannot pray for their salvation as they lost their opportunity (that is life). We cannot change the fact that there is one death and then judgement (unless of course, one raises the dead haha). So, in this same respect, the past is concrete. If prayers could change the past, or alter it, would there even be one soul in Hell?
Now, what can be done I suppose is a prayer for God to remove something from one's memory. If in Heaven we will not have sorrow and certain knowledge will be erased (so as to not cause mourning), could not God then, in this life, remove memories of the past or how they affect our heart?