Arjuna addresses the Lord as Madhusūdana because Kṛṣṇa once killed a demon named Madhu. Actually these questions, which are of the nature of doubts, should not have arisen in the mind of Arjuna, because Arjuna is a Kṛṣṇa conscious devotee. Therefore these doubts are like demons. Since Kṛṣṇa is so expert in killing demons, Arjuna here addresses Him as Madhusūdana so that Kṛṣṇa might kill the demonic doubts that arise in Arjuna's mind.
(Bhagavad-Gita---8:2---purport).

People propitiate demigods to satisfy their material desires. Those neophyte devotees of Kṛṣṇa who try to appease demigods like the sun-god in order to escape ill health do so because they succumb to serious doubts about Lord Kṛṣṇa's supreme divinity. In analyzing the word anyābhilāṣa ("desires other than those directed toward serving Lord Kṛṣṇa"), we find that one fosters this type of perverted intelligence when one thinks that the sun-god, who is merely a manifestation of the Supreme Lord's potency, can protect one from ill health but that the Supreme Lord, Kṛṣṇa, cannot. Once these mind—clouding doubts disperse, one enters the doors of pure devotional service. Karmīs and jñānīs are also tainted by material desires—the desire to enjoy their senses and the desire for liberation, respectively. Pure devotional service is attained only when these material desires are dissipated and one renders unbroken, favorable devotional service to the Lord Kṛṣṇa.
(Renunciation Through Wisdom).

At times, when doubts and restlessness assail us, we must remain fixed in our resolve. The best remedy for doubts is to seek the association of saintly persons. Saintly souls who are learned in the conclusions of the revealed scriptures and have realized the Supreme Lord can dissipate our doubts and calm our restless mind with unequivocal instructions and exemplary actions. When Kṛṣṇa conscious topics, which are both very potent and nectarean to the ears and heart, are heard and discussed in the association of saints, then faith in the Supreme Lord gradually increases, along with attraction and devotion to Him. Faith inspires initial surrender, and later, by the powerful influence of saintly association, one's faith deepens and becomes steady. Once faith becomes steady, all mental agitations and doubts clear up due to constant worship of the Lord. One then practices bhajana (chanting meditation) of a very esoteric and elevated nature, and this leads one to the stage of love of Godhead. To attain this state, saintly association is imperative; there is no substitute.
(Renunciation Through Wisdom).

The complication of materialistic life is due to an uncontrolled mind and uncontrolled senses. One should feel grateful to the Supreme Personality of Godhead for having obtained such a nice human form of body, and one should properly utilize it. The distinction between an animal and a man is that the animal cannot control himself and has no sense of decency, whereas the human being has the sense of decency and can control himself. If this controlling power is not exhibited by the human being, then he is no better than an animal. By controlling the senses, or by the process of yoga regulation, one can understand the position of his self, the Supersoul, the world and their interrelation; everything is possible by controlling the senses. Otherwise, we are no better than animals.
(Srimad Bhagavatam---3:31:19---purport).

There is one easy weapon with which the mind can be conquered—neglect. The mind is always telling us to do this or that; therefore we should be very expert in disobeying the mind's orders. Gradually the mind should be trained to obey the orders of the soul. It is not that one should obey the orders of the mind. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura used to say that to control the mind one should beat it with shoes many times just after awakening and again before going to sleep. In this way one can control the mind. This is the instruction of all the śāstras. If one does not do so, one is doomed to follow the dictations of the mind. Another bona fide process is to abide strictly by the orders of the spiritual master and engage in the Lord's service. Then the mind will be automatically controlled.
(Srimad Bhagavatam---5:11:17---purport).