Selfless devotion to Shri Krishna is the only way to purify the heart. So continuously practice devotion until your heart is completely purified. It may take one year, two years, may be one life or even many lifetimes. Guru will not initiate you with the divine mantra until your heart is completely purified. The moment he initiates you with his mantra, you will become free from the bondage of maya, all sufferings will come to an end and you will immediately realise God.