You can try to be the master of your senses but only Krishna can master your mind. In this world, devotees alone are controlling their minds because they engage them always in the service of Krishna.
The mind rules non-devotees. Devotees rule the mind.
If we contemplate on bad, selfish and egoistic thoughts, the mind will force us to speak and act accordingly in due course of time.
Prabhupada was asked, “When the mind is drifting, what should we do?” Prabhupada said, “Chant loudly!”
Lecture on "Srila Prabhupada Gave Us Everything" – SB 3.5.51 – 31st May 1996
Mind is one's biggest enemy when uncontrolled, but majority people put full faith on their enemy.
The more we feed the laziness of our mind, the more it becomes strong.