Source: Srimad Bhagavatham : Its Message for the Modern Man by Swami Shantananda Puri Maharaj
Srimad Bhagavatham (11:14:27)
विषयान् ध्यायतश् चित्तं विषयेषु विषज्जते
माम् अनुस्मरतश् चित्तं मय्य् एव प्रविलीयते
So long as a person goes on thinking of worldly things, he will get immersed in the world. So long as he goes on contemplating on the Lord, he gets immersed in the Lord who is the Ultimate Reality.
Please check these: Excerpts from a discourse by our Sri Sri Muralidhara Swamiji
The more you think about the world, the more your heart is filled with dirt and filth. The more you bring Sri Krishna into your heart, the more it is filled with bliss and peace. Chant the Mahamantra with faith and devotion.