The Greater Good or Regrets
Life used to be simple, right was right and wrong was wrong. No problem was so great that a pat on the back and a lemon drop couldn't fix it. But no, reminiscing is a dangerous path. This is a war, there is no time for the past, the regrets. He is the leader of the light, the mentor of the boy-who-lived-to-save-them. He cannot afford to dwell, to mourn, to grieve the loss of innocence. One does not win a war by planting peace lilies and handing out sweets, he has to make sacrifices. He has to manipulate, make decisions that he will regret even as he makes them for the 'greater good'.
Supreme Mugwump, Chief Warlock of the Wizengamot, Headmaster, twinkling-grandfather-like mentor, that is all the world would see; that is all he would allow the world to see. Hide Grindelwald, hide Ariana, hide the regret, hide the fact that he too is just human, fallible. That is the life he is condemned to as the man people look to, the man Voldemort fears. Ignore the collateral damage, ignore the pain his decisions cause, it is all for the best, keep repeating it until it is true. So that when he stands before Voldemort he can justify the means by the end.