उद्धरेदात्मनाऽऽत्मानं नात्मानमवसादयेत्। आत्मैव ह्यात्मनो बन्धुरात्मैव रिपुरात्मनः ॥

uddhared ātmanātmānaṁ nātmānam avasādayet ātmaiva hyātmano bandhur ātmaiva ripur ātmanaḥ

Let him raise the self by the Self and not let the self become depressed; for verily is the Self the friend of the self, and also the Self the self's enemy;

In plain words: Krishna says you must lift yourself up with your own higher self and not let yourself sink down. Your own mind can be your best friend or your worst enemy.