A person should lift himself up by his own self and not let himself sink. For the self alone is the friend of the self, and the self alone is the enemy of the self.
Explanation
Krishna is teaching Arjuna about the path of yoga (disciplined union of mind and self), and here stresses personal responsibility. The word for self (ātman) is used in two senses: the disciplined mind and effort that lifts one up, and the undisciplined mind that drags one down. Commentators differ on whether the "self" here is mainly the mind or the individual person, but all take the point to be that one's own inner effort decides whether one rises or falls.