B.G 6.05 and 06

उद्धरेदात्मनाऽत्मानं नात्मानमवसादयेत्। आत्मैव ह्यात्मनो बन्धुरात्मैव रिपुरात्मनः ॥५॥

uddharedātmanā'tmānaṁ nātmānamavasādayet। ātmaiva hyātmano bandhurātmaiva ripurātmanaḥ ॥5॥

One should uplift himself with the help of mind and should not degrade. Certainly, the mind alone is the friend of oneself, and also the enemy.

बन्धुरात्माऽत्मनस्तस्य येनात्मैवाऽत्मना जितः। अनात्मनस्तु शत्रुत्वे वर्तेताऽत्मैव शत्रुवत् ॥६॥

bandhurātmā'tmanastasya yenātmaivā'tmanā jitaḥ। anātmanastu śatrutve vartetā'tmaiva śatruvat ॥6॥

The mind becomes the friend of oneself, by whom indeed the mind is conquered with the help of the lord. But the unconquered mind turns hostile. Indeed, such a mind acts like an enemy.

Gīta Bhāshya 6.05 and 06

Striving is necessary to ascend to the summit of Yoga. 'ātmā' - mind; 'ātmanaḥ' - of the living being; 'ātmanā' - with the mind; 'ātmānaṁ' - the individual being; 'ātmaiva' - the mind; 'anātmanaḥ' - the unconquered mind of the individual.

The practitioner can ascent to the summit of Yoga only through striving, is stated by 'uddharet' verse.

Whose friend is the self? Now, the friend says, "The self is the friend."

The word 'ātmā' refers to 'mind'; the word 'ātmanaḥ' refers to 'of the living being'; the word 'ātmanā' refers to 'with the mind'; the word 'ātmānaṁ' refers to 'the individual being'; the word 'ātmaiva' refers to 'the mind'; So, indeed, the living being conquers the mind, by stationing the mind in the intellect. It is also said:

"The wise declare that the mind is the supreme cause."

"The mind alone causes bondage and liberation for humans."

"One should uplift the living being by the mind, not degrade it. The mind is both the friend and the enemy of the living being; there is no doubt."

"When the mind is conquered by the intellect of the living being, then the mind becomes a friend, elsewhere it becomes an enemy. Therefore, a person conquers with the strength of intellect, and then, with devotion towards the divine, thus he overcomes the demons Madhu and Kaiṭabha."

- stated thus in Brahmavaivarta Purana.

The word 'anātmanaḥ' refers to the unconquered mind of the individual, i.e. to one whose mind is unconquered. If the mind is not helpful, it is the same as non-existent. If a servant exists, but does not conduct himself as a servant, then he is as good as non-existent for a master, and the master becomes a non-master. Similarly, the mind becomes an enemy for one whose mind behaves like an enemy (and thus, he is termed as 'anātmanaḥ').