The guilt of never saying anything to either of them before they passed is so immense sometimes- and the anger I feel towards my mother at causing the destruction of my relationship with these two men who tried their hardest with her is a fury... . How on earth can I possibly forgive this? I am not sure that even attempting a relationship is even worth all the pain it causes.

Unfortunately what you describe is all too common in families where there is a BPD. My sister went through much the same thing, being genuinely frightened of my dad and thinking he would harm her until she was well into her twenties. (My mom put me in the same category as my dad so my experience was a bit different.) You are not at fault for what happened! I hope reading some of the stories here and seeing how a BPD parent turning some family members against others through lies and manipulation is unfortunately so common an experience among members here will show you that this behavior is a symptom of the disease and not the result of anything you did. There is no way you could have realistically prevented this behavior, or seen it for what it was, at the age you describe, in the circumstances you describe. I hope you can let go of the guilt. It might help to talk with a therapist about all this. Hopefully seeing others' stories will go a long way to convincing you that these events were out of your control as well. It's really sad that unrelated circumstances took your father and stepfather away from you before you could resolve things with them. I'm sorry for your many losses.
As far as forgiveness goes or building some sort of positive healthy relationship with your mother, that seems like it might be down the road a bit. There's no need to rush that part. It will happen when it happens. And if it doesn't, that's okay too. The first step is to separate what was your responsibility in your relationship from what was hers, and to genuinely let go of trying to make her anything other than what she is. It sounds like right now it may not be the right time to be in touch with her or try to work things out with her. Asking a BPD to accept responsibility for their past actions and talk through their effects on someone else is like asking water to run uphill. If you put the brakes on this effort, it will free up a lot of energy to try other ways of understanding and coming to terms with the past that are not as difficult.
Glad you found us here... . I think participating here will be very helpful to you... .