Disclaimer: JKR owns the Harry Potter world - I just like to play in it. Some parts of quoted dialogue come from HBP, written by JKR.
He looked down into the shallow bowl, the afternoon sunlight glinting off the silvery liquid. So many memories . . . Harry's angry face peered up at him, 'Snape's what happened! He told Voldemort about the prophecy -' His voice echoing in the silence. Dumbledore inserted his wand and twirled it around in the silvery substance. Another face rose to the surface; a long sallow face, with black eyes, 'Headmaster, you can't know what you ask . . . you take too much for granted!'
A sharp pain ripped through Dumbledore's chest. So much . . . it was all so much to ask of them. They were so young, both of them, and have already been through too much. Would either of them ever forgive him? Ever forgive him for what he was asking them to do? For what he would demand they do? Would they be able to forgive themselves? Could he forgive himself? He closed his eyes in exhaustion, rubbing his good hand over his face. How much longer could he last? How much longer until he finally got to rest. The liquid swirled again, and another dark-haired, young man's face drifted into view. 'Good evening, Professor. I'm just on my way back from Professor Slughorn, sir.' The cold eyes of the young man showed a brief flash of triumph before they were hidden behind long lashes. Another young man failed by Dumbledrore. No Tom, Dumbledore thought to himself, it's time your game were ended.
Dumbledore stood up wearily, picked up the pensieve and placed it in the cupboard. He latched it securely and walked over to his chair to collect his traveling cloak. Gathering his strength around him, as he gathered the folds of his garment, he straightened his spine in preparation for the task ahead. Forgiveness would have to wait . . . now it was time for the hunt. The tracking and hunting down of what bits remained of Tom Riddle's soul. Tom had already caused so much pain and destruction. Dumbledore would see an end to it . . . with the two people he loved the most in the world, who had been hurt the most by Tom . . . they would all see the end of this madness.
Dumbledore turned around, and walked toward the door of his office, he could hear Harry on the steps now. May he forgive them, may they all forgive him.