Hi neonpossum,
First, congrats on your diagnosis, and how you are handling it. You do sound like a really great guy, open minded and kind. And your motivation for wanting to tell your mother about your diagnosis is really good as well, wanting to apologize for your outbursts of temper when you were younger.
So really, good on you.
Borderline personality disorder is one of the most difficult diagnoses. I am thinking it might be far past what your mother could understand. Even if she did some reading on the net, she would be hard put to understand it. And maybe she wouldn't be able to accept that it is partly an inherited trait, and partly due to abuse. She maybe wouldn't want to know that what happened in her home was abuse.
Of course it was, neonpossum. I feel sorry that you had to deal with your stepfather.
I am thinking two things now. One, I am thinking that you can't change the past, you can only change the future. Maybe that's where you could aim your efforts at finding an understanding with your mother. Maybe regard the efforts you make in dealing with your condition and calming yourself and being happy as a way of apologizing. Of making up for your outbursts. Although, I think you have nothing to apologize for, nothing to make up, you were a teenager!! It's normal. Of course I don't know what happened.
The second thing I am thinking is because of a friend. He is an alcoholic, and just starting on the path of recovering. He has ADD, PTSD and maybe a bit of bipolar. He was abused by both his parents, and I see in him both an anger and a need for love from older women. I perceive him as letting the women in his life make too many demands on him, because he is needy for their love. And concurrently there is anger in him towards these older women.
I guess I am wondering if this is the case with you. You got angry at your mother in your teens and you feel a need to make up for it. You want something from her, affection, love, understanding? But she also subjected you to a bad situation when you were young - she put you in the power of an abusive stepfather. You had reasons for your anger. But you don't ask her to apologize. You think of apologizing to her.
Neonpossum, I am really really conscious that I don't know you well, I don't know your mind, and that I am reacting to your situation from within my own situation, thoughts and experiences. Which is to say, I don't think I am right! I wish I could just listen to you. I think it is the best therapy, especially with someone as clear as you. So I had these thoughts that I wrote above, but please don't think I believe I know what is best for you, or that I presume to know how your mind is working. Just really, throw it in the bin if it doesn't apply, I put no weight on it myself.
I wish you the best in life, a calm and peaceful heart.