We can never erase karma nor reverse it. Karma from past or previous manifestations can only be neutralized. If we have a negative residual balance of karma from past or previous manifestations it can be neutralized only by indulgence in good karma, positive karma in present life. There was no other shortcut method to erase bad karma.
The neutralization of bad karma from previous lives takes time. Something that got accumulated in many lifetimes, eradicating the same in present life is a monumental task. Only a hardened seeker like Swami Vivekananda can finally succeed doing so. Does it mean one needs becoming a Swami Vivekananda in real life to erase bad karma yes, field of spirituality demands so.
In search of God six years of age, I may have had a bountiful collection of bad karma from previous lives. How to erase it, neutralize it was the problem! It took much more than a decade for me to search the mantra for Prarabdha karma. We all know our shortcomings of present life and can eradicate the same. But what of accumulated bad karma of previous lives that we know nothing of!
The day I indulged in mantra for Prarabdha karma, journey of spirituality became easier. Cutting all bondages of previous lives simplified. One after another I could dismantle all bondages of previous lives so much so that I finally succeeded reaching stage of Nirvikalpa samadhi (absolute nothingness). Mantra for Prarabdha karma is not known to anybody world over.
It has to be realized from within. The moment mantra for Prarabdha karma dawns on us, all relationships that existed in this world disappeared overnight. We finally realized the underlying truth that we are primarily a spirit, soul atman on its cosmic journey that needs removal of dross impurities within. Indulging in karma nishkama karma way of life, we can finally demolish ego in totality. Demolition of ego means overpowering five senses and mind forever.
One finally becomes a free soul atman in the cosmic journey of life.