Is a person considered sinful for keeping a sum of money they found many years ago, using part of it, and giving the rest to their father, who used it to repair the building's door, and is this related to their life's difficulties?

If you picked up a lost item with the intention of owning it, not returning it to its owner, then you are considered a usurper and are permanently liable for it. You are absolved of this liability by returning it to its owner. If it is impossible to identify the owner after this long period, then donate it on their behalf or spend it on matters beneficial to Muslims. This applies if you were a legally responsible adult at the time of picking up the item; otherwise, your guardian is responsible. As for the condition you are complaining about, its cure lies in Tawhid (monotheism) and seeking forgiveness; in them there is healing, if done with sincerity and devotion.