When it comes to seeking closure from a person with BPD traits (pwBPD), our experience is that it's generally counterproductive. It's better to Gift yourself whatever Closure you feel you need. Remember that explaining yourself just exposes yourself to needless discord. We have to remember that using
JADE (Justify, Argue, Defend, Explain) is a communication approach that works well with us reasonably normal people, typically fails with pwBPD. In other words... if it's over, then just accept it's over. Let the sleeping dogs lie undisturbed.
Some here have really felt a need to DO something. How about writing your letter and then, with or without friends or ceremony, let it burn. (No forest fires, please!)