In a talk Elder Russell M. Nelson gave during the October 1995 General Conference, he suggested if were to ask which of the Lord's commandments is the most difficult many would cite Matthew 5:48, "Be ye therefore perfect, even as your Father which is in heaven is perfect." LDS doctrine is unique on this point in that the Savior's words are not considered to be symbolic but a true commandment.
Realizing that the Savior actually expects us to be perfect can be the source for extreme anxiety. Converesly, Elder Nelson's words were very comforting. He explained the Greek text from which the King James verson was translated used a word that was not necessarily about errorless performance. Teleios means something more along the lines of complete. Although we are to aspire to perfection, we cannot actually be complete until after the resurrection. Our mortality, something that is far beyond our control, is by itself a roadblock to perfection. The Savior's sacrifice removed this roadblock for everyone.
Of course, life is the pursuit of perfection - or completness. On my spreadsheet of life projects. Project name: Perfection. Project status: Pending.