Classical Vedanta · part of the Mahabharata
Bhagavad Gita 6.5
The mind is both your closest ally and your worst adversary
उद्धरेदात्मनात्मानं नात्मानमवसादयेत्। आत्मैव ह्यात्मनो बन्धुरात्मैव रिपुरात्मनः॥
Elevate yourself through the power of your mind, and not degrade yourself, for the mind can be the friend and also the enemy of the self.
What this verse points to
The same instrument does both jobs. Nothing else has to arrive for the mind to turn against you, and nothing else has to arrive for it to lift you — which means the deciding factor is how it has been trained, not what happens to you.
The verse puts the responsibility somewhere uncomfortable. Not on circumstance, not on other people, and not on a future in which conditions improve.
Sit with it
Sit and simply label, for two minutes, whether each passing thought is working for you or against you. Do not argue with any of them.
About the Bhagavad Gita
The Bhagavad Gita is a 700-verse dialogue set on the battlefield of Kurukshetra, in which Krishna counsels the warrior Arjuna at the moment he refuses to fight. Across eighteen chapters it draws together the paths of action, devotion, and knowledge into a single teaching on how to act without being bound by action. It sits inside the Bhishma Parva of the Mahabharata and has been the most widely commented-upon text in the Vedantic tradition, from Shankara onward.
References to this text are given as chapter.verse — here, 6.5.