द्वे हुडाविव युध्येते पुरुषार्थौ समासमौ ।
प्राक्तनश्चैहिकश्चैव शाम्यत्यत्राऽल्पवीर्यवान्॥

There is a conflict going on between two unequal entities, namely, the result of actions in a previous birth and the result of actions during this birth. The fight between these two is like the fight between two fierce birds or animals. In this fight, the one which is weak will surrender to the one which is strong.
Effect of past life karma will remain there however, One should, by Herculean effort and gnashing one’s teeth, gain victory over the bad effects of the past karma by the good karma done during this birth.