I wrote a letter to my abusive narcissistic sister about a decade ago. Not to send it, just to vent.
My hubby, sick of watching me get mistreated over and over again, put it in an envelope and sent it!I was mad, but it was speaking honestly about my feelings about our relationship and her behaviors which were hurtful. I didn't hear from her for a long, long time...and I was glad. It gave us both time to get help.
We're not close, but we see each other once or twice a year. She's never mentioned the letter and I feel no obligation to express any remorse. She has stopped doing several of the things she used to do, but she's also done others which were far worse.
It was worth it.