Regret is a normal part of life. What if? Why did I? Why didn't I? Is it my fault?
Some of us are more neurotic than others and it is fine that we are not all the same. We think differently because of our upbringing, background, personality and gender and our unique experiences of guilt, shame and fear.
We can do things about our regrets. First, what is your intention? Peace of mind or release of guilt, anger, shame? Clarity or healing? Saying "goodbye"? It is best not to assume too much when considering your actions.
First of all you can talk to the person directly. This takes bravery and energy. Your motivation should be kindness. Be prepared to see your own issues reflected back! Consider the other person's point of view. What can you learn?
Secondly: you can write a letter to the person. This way you can reflect and be mindful. Be prepared fot the reaction and again, do not assume. Say what you feel. Imagine dying and writing to a loved one and how a letter would be a loving memory and explenation for them to absorb at their own pace.
Finally; you could write a letter and then release it by burning or burying etc. This way there is no obvious action. You could even read it to a third party for their thoughts on it. If you want more peace in the world then make peace with yourself. Forgive.
Take care, lots of love.