When Nemo went with his mother his father killed himself
His wife cheated on him and left him and his son chose his mother. Odds are he was at a very low point.The 118-year-old Nemo is the one from the timeline where he insulted Anna.
In this timeline, at this point, he would regret his action and think about what would have happened if he had admitted his inability to swim, and it all sort of snowballs.