Closure in a Relationship/friendship

I have “unfinished” business in a past friendship. I don’t want to be friends with this person but I don’t like the way we ended things. Is this something to work out on my own with my worn thoughts (i.e. work on the thought “I don’t like the way we ended things”) or is it ever worth re-engaging someone? What is the point of the letters from the relationship month? Why ever send them to someone you don’t intend to continue to engage in a relationship with if the relationship is just in your own mind a la what Brooke teaches?