Concentration is not thinking of one thing. On the contrary, it is excluding all thoughts, since all thoughts obstruct the sense of one's true being. All efforts are to be directed simply to removing the veil of ignorance. Concentrating the mind solely on the Self will lead to happiness or bliss. Drawing in the thoughts, restraining them and preventing them from straying outwards is called detachment (vairagya). Fixing them in the Self is spiritual practice (sadhana). Concentrating on the heart is the same as concentrating on the Self. Heart is another name for Self.
Ramana MaharshiPlace your burden at the feet of the Lord of the universe who is ever victorious and accomplishes everything. Remain all the time steadfast in the heart, in the Transcendental Absolute. God knows the past, present and future. He will determine the future for you and accomplish the work. What is to be done will be done at the proper time. Don't worry. Abide in the heart and surrender your acts to the Divine.
Ramana MaharshiThe greatest obstacle to enlightenment is getting past your delusion that you are not already enlightened.
Ramana Maharshi