One's past in a specific lifetime, in general, can not be let go of. However, the pull of those past events can be lessened provided one embrace the concept of karma and reincarnation. Usually because of karma, they can not be fully released because if the events hinges on, lets say, a girl is being hurt by another girl's actions to her significant other, either in a past or future life the girl being hurt was/will incarnate as the other. But, when one is in their last incarnation for becoming a, let's say christ, after feeling the pain of the other body your past life's action hurt you can embrace it and relinquish the pain of it. In that case it becomes a part of their testimony for their conscious level.
Sonetimes, though, when both events are not in before becoming christ incarnations they do forget it after separating from the other person. This is done so that during the incarnations between the "reaping what was sown" they will not be hampered by the event. Other times they will reflect back on it on occasions for burying it deeper into their life-force because of how deep the other incarnation's reactions of hurt are.
Even this explanation can not do anything to lessen the pains karma has in store for us, it must be reaped completely as it was with other life-force living it.
Be At Peace with all things as much as Karma will allow.