The ability to overcome failure--to live through it and move on--is crucial. If we are not willing to face failure--if we don't have the skills to survive it--we have precluded any real creativity or risk. Failure may never become our friend, but if we are to do meaningful work, perhaps failure needs to be our companion.
We have to accept that any action we take might promote an equal and opposite reaction that we do not want. We have to realize that even the most noble actions or most obviously correct course can have its dark side that we cannot control or reason our way out of. The fighter of the "just war" must understand that her actions will result in the deaths of other humans; many of whom may be innocent. The pacifist who refuses all war must realize that his inaction might likewise result in the deaths of the innocent. There are no actions without contradiction—and yet we must act, for not to act is also a contradictory action with both positive and negative effects.