A letter to Abdullah bin Abbas. Ibn Abbas said that except the advice and sayings of the Holy Prophet (s) no other advice benefited him more than this. A variation of this letter has appeared earlier on - see Letter 22.
Sometimes a man feels very happy for getting a thing which was actually destined for him and feels sorry for not getting a thing which he was not destined to get. Therefore, you should not feel happy if your desires are fulfilled and should not feel sorry for deprivations. (Leave everything to the Will of Allah). You should not consider the achievements of the pleasures of life or opportunities of revenge for wrongs done to you as the main blessings bestowed upon you but you must feel happy for good deeds that you have provided for the Hereafter. You must feel sorry for the wealth that you have left without spending it on good causes and the opportunities you have lost of doing noble and pious deeds. Nothing should worry you but the idea of death and the life after death.