Hello and welcome to the family. I'm so sorry you're in this mess and hopefully we can find a few words of wisdom to help you through this.
The BPDs in my life are an ex-wife and a daughter, but in hindsight I always suspected my mom as well since she would explode over the tiniest things. And I realized that even though she wasn't diagnosed, the principles you'll learn here work whether someone has BPD or not. So let's talk through a few of those.
First, someone with BPD has trouble with boundaries- like your mom blaming things on you and then wanting to be best friends the next day. That's not how life works, and you need to start making that clear. If she berates you on the phone, tell her she's being unfair and explain that you have to go and will talk to her another time. You don't have to be mean about it either- it just has to be consistent.
For instance, she calls and starts ranting about your childhood, maybe give her one warning...mom, I don't want to talk about this. If she continues, tell her that you're not having that conversation and you have to hang up. That's a boundary and it's so critical to deliver it in a very predictable way every single time.
Now, your mom doesn't like boundaries, so she will not respect your wishes (at least at first). She'll call, text, and email with suicide threats, because when she does that, you come running to give her the attention that she needs. So that brings us to boundary #2, if mom says "suicide", you dial 9-1-1 and have her taken off in an ambulance.
Again, you do this each and every time in a very predictable way. You scream, I'm getting off the phone. You threaten suicide, then I'm sending emergency response.
Your mom will try to blame you and make this all about you- but go back to boundary #1. You're not doing that anymore. You can explain to her though that you're only reacting to her behavior and you've made your intentions very well known. You want a relationship with her but it has to be reciprocal. All the decisions are hers to make- be nice and have a relationship, or be ugly and have no relationship.
Again, I'm so sorry you're in this position and so many here can relate since it's very similar behavior patterns. You mentioned until you were mid-20's, all your mom's venom went to your dad. A better way to say that is she's needed someone to blame her entire life in order to remain a victim. You left, just like your dad left, and you got the same amount of blame for putting yourself first.
Please continue to share and ask questions. Hopefully this helped as a starting point.



. If she had moved on with someone completely unrelated I wouldn't have been as hurt and I definitely wouldn't have cared as much. It would have still hurt but way less than this. And to answer your question, if the roles were reversed, YES. I would 100% feel like I cheated. Not only this is someone I introduced her to, this is someone I was jealous of, someone I didn't like (and she knew the whys) and someone who always felt inferior to me for no reason at all. Also someone she very clearly got closer and closer until I said enough. This guy is genuinely soulless and fake, he is a super fake person I'm not sure how and why she'd fall for him. I warned her, too. Still fell for him.