Hi Yellowballoon

Welcome!
I'm so sorry for what you are experiencing here. There is a lot to unpack in your story. It's great you have a T, and supportive family, and bf. You also clearly have a reasoned analytical head on your shoulders, and as an adult you can see the issues now in ways that you couldn't see as a child or teenager. It is fantastic that the veil has lifted and you are seeing the issues. What you are hoping for now is to figure out how to navigate and stick handle your way through this in a way that you can remain whole and emotionally healthy. Am I close to having that right?
First of all, congratulations for your clarity and seeking support. I didn't get to where you are until my fifties. So I have a lot of admiration for you.
What can I do? I'm so exhausted and frustrated but I love and miss my dad. I also feel a lot of guilt for distancing myself from the wife because she does have BPD. My dad reminds me of this every time I try to talk to him. "It's not her fault because she has BPD." Am I giving up on her when I should be trying harder to support her? I don't think so at this point in my life, but I still feel guilty. I need to know if I am just not productive in the way I interact with her. I'm fully sympathetic to the struggle a person with BPD goes through daily, but I think there must be a point where I can say, this is the line and it's ok that I'm separating.
We get it that you love and miss your dad. Of course you do, he's your dad, and the loss of your mom made him even more important. No, it's not step-mother's fault she has BPD, but regardless of what circumstances contributed to that, she is accountable for her actions as an adult. Yes, you absolutely should draw a line in the sand, and separate yourself for your own emotional safety and well-being when that line is crossed.
My 87 yr old mom is uBPD (waif type). I am almost 61. My father died about 18 years ago. I
now hold him in hero status but when he was alive, I didn't know how much he must have put up with. I believe he protected me from that. My childhood with my mom was marked with unpredictability, and turmoil. I have major trust issues. I spent a lot of time in the trees behind my home. That's where I went when I needed respite from her. I too was an only child. Dad supported my mom, but I really can't say if he enabled her. He was a work-a-holic so he wasn't always around. It may have been his escape. But I do have many memories of him saying things like "why does there always have to be a crisis?" "Why do you always have to twist things?" "That's enough Jane!" (not her real name) in an authoritative voice (which wasn't at all normal for him). He was a kind person and very patient, and did hold her to account to some degree when she crossed a line. She held a knife up to him just after they were married, and he told her if she ever did that again the marriage was over. She was smart enough to not cross that line again. He was a man of his word.
Fast forward to today, and the only way I can safely function in the same town as my mom is to hold a hard boundary for space. By this I mean that I came out of retirement and went back to work to avoid her making me her personal caretaker for all her needs including meals, bathing, house cleaning, emotional punching bag, you name it. If I said I couldn't do something, then she would say "You don't love me!" It didn't matter how much I did, it was never enough. She's very weak and started falling. After her first fall, I was there 3-5 hours a day trying to support all her needs. That was WAYYYYYYYY too close. I was a wreck.
It was a process for me to stop justifying myself, stop explaining, stop arguing how unreasonable she was, and stop defending myself. The more I defended myself, the more she attacked.
When I came out of retirement and went back to work, she went nuclear.
That was 14 months ago, and I can tell you that taking care of myself by ensuring I have space from her was essential. Today, my relationship with her feels superficial. She has replaced all the tasks (physical and emotional) I used to do, with friends and paid workers. I let her figure it out, because any suggestions I would have had would have boomeranged back in my face in an emotionally harmful way. I see her once every week or two, usually in the company of my H or D26. My H buys her groceries every Wednesday, and takes her to Dr appointments. Above and beyond that, she has found other caretakers. She texts and I choose which texts to answer; the others I ignore. Last weekend I surprised her by picking up two ice cream cones for the visit. This put her instantly in a good mood, and I got her going on an activity, and then left after an hour. It's like when the dentist gives the child a prize for the visit to the dentist. My mom
has adjusted to me working again. Trust me, they are very capable of finding ways to adapt to "have their needs met" by other people. They use manipulation, and charm, and all kinds of tools. She is really a child in an adult's body, and she has found numerous people who "feel good" by helping her.
I had to stop wishing my mother could be the mother I so desperately wanted. I wanted someone who would love me unconditionally for being me. I just couldn't believe this wasn't my mother, because sometimes she was ok, or even better than ok. The problem was the unpredictability and her sudden rages. I couldn't accept that she was someone who would only love me if I "supplied all of her needs". In the case of your dad, he is not only enabling his wife, but also using you in a way that is harmful to you, but makes his life easier at home with his wife. This is not ok. And yes, you should have a line in the sand. And until you do, nothing is going to change or get better. She uses you to dump all her negative emotion on so she can feel better, and he uses you so she has less negative emotion to dump on him. He is probably afraid of her. I was terrified of my mom. That she physically attacked your dad is another whole level of escalation. Getting in between them, and taking her to a "safe place" when he threatened to call the police also enabled her. But in the moment, you did what you thought was right. Now you can see the dysfunction. Trust me when I say you can never fix those dynamics, any more than you can stop the tides from changing, or the rotation of the earth. This is a job for a psychiatrist, and they will tell you that BPD is hard to treat. The pwBPD often stops attending, and will likely blame the psychiatrist.
It is not your job to "rescue" either of them from their emotions or fear or anything else. They are adults, and need to figure this out in their own way. They are enmeshed, and each getting something from the relationship, even though you may not be clear what it is. If your dad wasn't getting something from it, he would leave. You also mentioned he has a history of being attracted to women who don't treat you well. The only thing that is going to help you here, is using the principle of natural consequences. You can't help or rescue them from their enmeshment.
You already know all this, but still when I was at the point where I landed on this forum, I knew something was very very wrong, but hearing it from others still helped me because it was validating. pwBPD keep telling us we are the selfish ones, "what's wrong with us?", "why are we have to hurt their feelings all the time?" The gaslighting was awful.
So when I finally accepted that was who my mom was, and she wasn't going to change, and I accepted it was me who was going to have to change, that was the turning point. Then, I had to figure out how to do that. For me, returning to work out of retirement was one way, because that was somehow socially acceptable, since I wasn't available. I live in a small town where everybody knows everybody. I grew up in this town, so people who know her also know me. My career was in this town, and I had a good reputation, so that helped me a lot. But like you, I still felt intense "guilt" for not being able to "supply" my mom's "needs". The guilt is the kicker. Society sees an elderly person, weak and vulnerable, and expects family to step up. My mom has refused to even consider viewing an assisted living facility, and cancels the home care I arrange for her after she starts to recover from one of her falls. I feel the judgement of society intensely. But I let the chips fall where they may. If I am at work, her friends and hired workers can supply her needs. Eventually they will kind of figure some things out, because she is incapable of holding back on her emotions. One of them once showed me a text she had sent: "I waited for you all day".
Still, I have figured out a way to have a relationship with my mom. I haven't given up on it. I totally get it that you still love your dad and want a relationship with your dad. This is not the relationship I wanted or thought I would have with my mother, but it's one that I've adjusted to and can live with given the circumstances. It's the least of all evils.
I think it's great you've already withdrawn and taken steps to protect yourself from your father's wife. In terms of pragmatics, I would work on finding ways to not let her in your house alone, or maybe you've already done that. Your bf should always be there, or if she wants to see you, meet in some public space for a coffee. They don't mistreat you the same way when other people are around. And definitely limit the interactions, and continue to reduce them over time.
A big step for you may have to be to accept your dad for where he is right now. He's shown repeatedly that he's more concerned about her needs, and also his needs, rather than yours. "Meet him where he's at". And allow yourself the space to grieve through that. It's all very difficult.
For me it was helpful to balance out the grief process by also redirecting my thinking to focus on the positives. In your case, you have family that has some understanding, and supports you. You have a T. You have your bf. Make a list of all the things you are grateful for in this world. I found that helps to balance the loss, even if just a little bit.
I'm sorry if this is too long. But I could hear your pain, and just wanted to say I think you are very much on the right track in figuring all this out, and I am hoping that by sharing a little of my own experience, there might be a thread that could be helpful as you continue on this journey. Hugs my friend

, and again welcome to our forum.