Hi Motherless Child

I'm so sorry that you're experiencing this awful situation. I too have been there.
My mother is unBPD and dad is an enabler/NPD.
We have been NC for almost 2 years, but recently they have sent me guilt-ridden emails and have tried to pull me back into their crazy world.
For my dad, who always lets mom run the show, I wrote out healthy guidelines with my T that, if he agreed to them, would make it possible for us to have LC. These guidelines were
1) Our relationship is sacred without mom
2) You're my dad and will refer to yourself as dad (he always uses his first name with me... .don't ask why, I have no idea, other than to make things even less intimate)
3) Our conversations are private
4) My relationship with Mom is between the two of us, you will not get into the middle of it
5) We will not betray one another
Doesn't seem like that much to ask, but he said that he had to share everything with her, even to have her on the phone if we were going to talk.
Then she wrote me one more email, dousing guilt that my son's 7'th birthday had just passed and they missed him. She asked to meet with me and I replied that if she took the time to start seeing a therapist to work on herself then I would be willing to meet with her.
She replied with "Message received. Lord bless and keep you."
Then I get this email from my dad... .
When (unBPD mom) asked me if I thought it would be worth while for her
to extend her offer to meet with you again, my
response was, "It can't hurt". Boy, was I wrong.
We've loved you from the minute you were born (which I guess is
why it hurts). We did our best to raise you in a loving home
(obviously, it was severly deficient from your perspective). We
sacrificed and willingly invested many years in trying to help you and
provide for your needs (as I know are doing for (your son) and (your daughter).
Obvoiusly, you no longer need us. Your heart has become hardened
towards us.
The reason for writing is to ask you why. It is becoming clearer
that this is probably the way it is going to be. I've seen this
happen in other families but never thought that one of our children
would ever reject us like this. I was blindsighted. We have lost our
daughter, our first grandson and only granddaughter. I think we at
least deserve to know why. I would hope (your daughter) would do the same for
you.
Stupid, clueless, worthless, demeened, and discarded father
No guilt there, right?

I replied with... .
Dear Dad,
I'm sorry that you've taken my message as rejection. I can assure you that my heart is in no way hardened. I see value in private therapy and believe that mother would greatly benefit from it, as I have, which is why I suggested it to her. There was no malice or intent to hurt in my suggestion.
I haven't discarded you. I outlined guidelines for a safe adult-to-adult relationship with you, and sadly you declined.
We got to this place due to the many negative verbalizations from mother about my husband and my life and then she threatened to "cut all ties" with me if I didn't change a business decision that involved my brother. I will not be bullied and belittled like a child. That is the reason why.
Yes, you have lost the scared little girl (Jenny-A) who was your daughter. In her place is a grown up woman (Jenny-A) with many wonderful qualities, one of them being a healthy respect for herself and standards for how others should treat her.
I sincerely hope that you both put time and effort into private therapy to gain perspective and healing in your own lives. It is my belief that only then would our family have a chance to be put back together. Therapy is a wonderful, sometimes painful, but very rewarding experience. I know it takes courage.
Lovingly,
Jenny-A
And then of course I heard nothing back. Because they never have and never will take responsibility or even acknowledge that they have done wrong. In their minds they gave me everything and I am wrong to stand up to their constant verbal and emotional abuse.
As far as what you should write to your mom... .just know that you're never going to get the response that you want. She will feel rejected and you will be the bad guy.
I see a lot of value in writing letters that express every single thing that you've experienced and then don't send them. Those letters are for you. You could post them here. I just posted a letter that I wrote to my dad but never sent.
Working with Children, Parents, or Relatives with BPD / [L5] Coping and Healing in a Family with a BPD Parent, Sibling, or Inlaw / Letter to my BPD/NPD dad
Lots of love to you. I feel your pain.