Second Nephi 2:5

And men are instructed sufficiently that they know good from evil. And the law is given unto men. And by the law no flesh is justified; or, by the law men are cut off. Yea, by the temporal law they were cut off; and also, by the spiritual law they perish from that which is good, and become miserable forever.

We cannot save ourselves. We cannot keep the law perfectly. Sooner or later we fall short. The good news is that Christ never falls short.