As people, we are an amalgam of biology and experience.
Biology is directly deterministic. Experience is not.
The possession of a particular set of data and the interaction of this set of data with a person’s biology is what will shape their personality, cognitive ability, tastes, choices, and everything that defines them as an individual.
What experiences do is insert data into the system that is us.
The person we are is a consequence of the experiences to which our biological entity has been exposed.
So the person we are after a (striking? special?) experience is different from the person who initiated the action, who was on the receiving end of that experience.
This is why it makes no sense to feel regret for the mistakes of the past.
Learn from them, yes. That’s an imperative. It is a tragedy when we don’t learn from the past. But to feel regret over what was done?
What is the point of regretting something that was done by someone else?