bandhuh aatmaa  aatmanah tasya

 yena  aatmaa eva aatmanaa jitah

To him who has conquered his (base) self * by the (divine) SELF, his own self is the friend.

anaatmanah tu shatru tve 

varteta  aatmaa eva shatruvat

To him who has not subdued the  self, his own self acts as the foe

-Bhagavat Gita ( Ch.6 verse 7)

* ISKCON  publication replaces ' self ' with ' mind ' 

Swami Chidbhavananda's commentry

The base and the divine are the two mutually opposing natures at work in man.  When either of them predominates the other gets vanquished.

Where the body, mind and senses are under perfect control, the divine nature prevails and pulls the man up. Progressively he evolves into high orders of existence.**

But when he submits to the base nature it ruins him completely. The foe outside hurts (or attempts) only occasionally. The foe of the base nature on the other hand, is constantly causing havoc. The reckless man therefore is personally responsible for the evils he has brought on himself. He is the worst enemy of himself. 

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** The following quote is  not part of  Sri. Chidbhavananda's commentry.

Then the devil took Jesus to a very high mountain and showed him all the kingdom of the world in all their greatness. "All this I will give you, " said the devil, "if you kneel down and worship me." Then Jesus answered, "Go away Satan ! The scripture  says, 'worship the Lord your God and serve only him !' "  Then the devil left Jesus and angels came and helped him.

                                         - Matthew 4.8-11

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