Here's a really amazing excerpt from Srila Gour Govinda Swami's book, Amrutera Tarangini - The Flow of Nectar.
"Somehow or other we have come to this path, so we should be serious. Our Guru Maharaja Srila Prabhupada says, 'This is for serious students.' It is for those who are very serious like Dhruva Maharaja. Dhruva was so serious and determined. 'I must get Narayana.' So be serious and determined like him, then in this very life we will get Krsna. But if there is no seriousness and determination, rather, so many material desires, still whatever little we have earned is never lost.
"There is no despondency or pessimism on this path. This path of bhakti is all optimism. Never become disappointed. If it will not be achieved in this life, definitely in the next life or the life after that it must come to us. This path of bhakti is all gain, there is no loss. We may commit some aparadha, fall down or whatever, so in this very life our advancement is checked. But whatever we have earned is with us, it is never lost.
"There is no loss, it is all gain. So there is no pessimism. This bhakti is so powerful, and this is the teaching."