About those past misdeeds that we have performed but cannot recall, we can contemplate like this: “All the misdeeds that I have committed since beginningless time,whether intentionally or unintentionally, are all wrong. As if they are poison I have taken, I feel the greatest fear and regret for my action, and vow never to commit them ever again.” In this way, all negative karma can be changed into mutable karma. The significance of such resolution cannot be overstated. Otherwise, any karmic effect will be possible if this is not done.
[Excerpt from Luminous Wisdom Book Series ~ The Right View : On Cause and Effect]