SB 10.87.33

vijita-hṛṣīka-vāyubhir adānta-manas tura-gaṁ
 ya iha yatanti yantum ati-lolam upāya-khidaḥ
vyasana-śatānvitāḥ samavahāya guroś caraṇaṁ
 vaṇija ivāja santy akṛta-karṇa-dharā jaladhau
Synonyms: 
vijita — conquered; hṛṣīka — with senses; vāyubhiḥ — and vital air; adānta — not brought under control; manaḥ — the mind; tura-gam — (which is like) a horse; ye — those who; iha — in this world; yatanti — endeavor; yantum — to regulate; ati — very; lolam — unsteady; upāya — by their various methods of cultivation; khidaḥ — distressed; vyasana — disturbances; śata — by hundreds; anvitāḥ — joined; samavahāya — abandoning; guroḥ — of the spiritual master; caraṇam — the feet; vaṇijaḥ — merchants; iva — as if; aja — O unborn one; santi — they are; akṛta — having not taken; karṇa-dharāḥ — a helmsman; jala-dhau — on the ocean.
Translation: 
The mind is like an impetuous horse that even persons who have regulated their senses and breath cannot control. Those in this world who try to tame the uncontrolled mind, but who abandon the feet of their spiritual master, encounter hundreds of obstacles in their cultivation of various distressful practices. O unborn Lord, they are like merchants on a boat in the ocean who have failed to employ a helmsman.
Purport: 

To become qualified to attain love of Godhead, the mature fruit of liberation, one must first subdue the rebellious material mind. Though difficult, this can be achieved when a person replaces his addictions to sense gratification with a taste for the higher pleasures of spiritual life. But only by the favor of the representative of Godhead, the spiritual master, can one gain this higher taste.

The spiritual master opens the eyes of the disciple to the wonders of the transcendental realm, as indicated in the Gāyatrī prayers by the seed mantra of divine knowledge, aiṁ.

The Muṇḍaka Upaniṣad (1.2.12) states:

tad-vijñānārthaṁ sa gurum evābhigacchet
 samit-pāṇiḥ śrotriyaṁ brahma-niṣṭham

“To understand these things properly, one must humbly approach, with firewood in hand, a spiritual master who is learned in the Vedas and firmly devoted to the Absolute Truth.” And the Kaṭha Upaniṣad (2.9) declares:

naiṣā tarkeṇa matir āpaneyā
 proktānyenaiva su-jñānāya preṣṭha

“This realization, my dear boy, cannot be acquired by logic. It must be spoken by an exceptionally qualified spiritual master to a knowledgeable disciple.”

Non-Vaiṣṇavas often disregard the importance of surrendering to a spiritual master who stands in an authorized line of disciplic succession. Relying instead on their own abilities, proud yogīs and jñānīs exhibit their apparent success to impress the world, but their glory is but temporary:

yuñjānānām abhaktānāṁ
 prāṇāyāmādibhir manaḥ
akṣīṇa-vāsanaṁ rājan
 dṛśyate punar utthitam

“The minds of nondevotees who engage in such practices as prāṇāyāma are not fully cleansed of material desires. Thus, O King, material desires are again seen to arise in their minds.” (Bhāg. 10.51.60)

On the other hand a humble, steadfast devotee of Lord Viṣṇu and of the Vaiṣṇavas is assured of easy victory over the stubborn mind. He need not concern himself with performing the eightfold system of yoga or taking other such measures to keep his mind steady. Sarvaṁ caitad gurau bhaktyā puruṣo hy añjasā jayet: “A person can easily obtain all these goals simply by being devoted to his spiritual master.” Otherwise, a nondevotee may conquer his senses and vital air and still fail to tame his mind, which will continue to run wild like an unbroken horse. He will suffer unending anxiety over the troublesome execution of various spiritual practices, and in the end he will remain just as lost in the vast material ocean as he ever was. The analogy given here is very appropriate: A group of merchants who hastily enter upon a sea voyage with expectations of great profit, but who fail to hire a competent helmsman for their boat, will simply experience great difficulty.

The Bhāgavatam declares the importance of the bona fide spiritual master in many places such as this verse from the Eleventh Canto (20.17):

nṛ-deham ādyam su-labhaṁ su-durlabhaṁ
 plavaṁ su-kalpaṁ guru-karṇa-dhāram
mayānukūlena nabhasvateritaṁ
 pumān bhavābdhiṁ na taret sa ātma-hā

“The human body, which can award all benefit in life is automatically obtained by the laws of nature, although it is a very rare achievement. This human body can be compared to a perfectly constructed boat having the spiritual master as the captain and the instructions of the Personality of Godhead as favorable winds impelling it on its course. Considering all these advantages, a human being who does not utilize his human life to cross the ocean of material existence must be considered the killer of his own soul.” Therefore the first business of one who takes human life seriously is to find out a spiritual master who can guide him in Kṛṣṇa consciousness.

Śrīla Śrīdhara Svāmī prays:

yadā parānanda-guro bhavat-pade
 padaṁ mano me bhagaval labheta
tadā nirastākhila-sādhana-śramaḥ
 śrayeya saukhyaṁ bhavataḥ kṛpātaḥ

“O transcendentally blissful guru, when my mind finally achieves a place at your lotus feet, all the tiresome labor of my spiritual practices will be finished, and by your mercy I will experience the greatest happiness.”