Hi AppeaseNoMore and

To answer one of your later questions sooner...
Has anyone experienced something like this?
the nice thing about this group is that the answer is Yes, many members here have gone through similar situations; you're definitely not alone. While there are no "magic solutions" or "quick fixes", this is a place where you can learn the non-intuitive skills, tools, approaches, and most importantly, mindset shifts, that could help make your relationship more livable for you.
If I'm tracking with your current situation, the short version is that you'd appeased to stop conflict for quite a while, and have recently started trying something new (not reflexively apologizing). She is noticing a difference and is communicating that in unhelpful ways.
Couple of thoughts.
One is that it is healthy to decline to be the audience for extreme rants/emotional dumping. Nobody is helped, and the relationship is damaged, when she chooses to use abusive language to you, and you choose to be there to hear it. So, it makes sense to decide for yourself what your
true boundaries are (note -- true boundaries are not ultimatums, rules for other people, or necessarily verbal statements. True boundaries are rules that you decide for yourself, that are 100% under your control).
Prior to this, I knew very little about BPD, I'd heard the term but that's about all. I had no idea how to handle someone like her, and hadn't read other people's experiences of appeasing which inevitably seem to go wrong in the end. I'm now trying to set limits (I won't engage in one-sided discussions about my "problems", I won't take her interrupting me to put words into my mouth, etc). I've told her that I think she has been too dominant in handling disputes, and at times has crossed the line into what I think is verbal abuse.
We can refine your ideas a bit, and you can see if it's helpful.
Sometimes when dealing with a pwBPD, it can be much more helpful to
do our own boundaries, than to
explain them to others. In fact,
JADE-ing can sometimes make things worse. So, I wonder if instead of trying to explain to her that she's been verbally abusive, you could decide for yourself that you won't stay in conversations where you don't like how you're being spoken to?
Maybe a previous example would've been:
Her: "AppeaseNoMore is a &*%%^$^ piece of *$%@#%$@^ and never listens and is a horrible person and blah blah blah"
You: "I won't tolerate being spoken to that way. You're being verbally abusive. It's wrong and I will not accept it. You need to stop talking to me that way."
Her: "Actually, I'm not being verbally abusive, you are, you're the problem, I can't believe you always blah blah blah"
...
A possible tweak for the future could be:
Her: "AppeaseNoMore is a &*%%^$^ piece of *$%@#%$@^ and never listens and is a horrible person and blah blah blah"
You: "I don't like hearing that. I'm going to the store; I'll be back in an hour."
Her: continues saying whatever, but you aren't there to hear it
...
Basically, doing instead of explaining boundaries minimizes the possibility of continuing the conflict (explaining tends to open more doors for argument) and protects you from prolonged exposure to hearing awful things.
...
Another thought:
As odd as it sounds, she's right that you're interacting differently. She may not be expressing herself skillfully about that, but she is correct.
That's where validation comes in. Validation and boundaries go hand in hand. One isn't better or worse than the other, just like a hammer isn't better or worse than a screwdriver. It's all about which tool/approach is appropriate for the situation. We don't recommend validating abusive language -- that's where boundaries will apply. Boundaries are for your protection.
Validation --
true emotional validation -- is a way to connect safely. Like you said, you want to make this relationship work, and an "all boundaries" relationship sounds pretty sad and isolated. Typical approaches to connection that might work for "generally normal" relationships might not for a BPD relationship, which is where the skill of validation comes in.
Validation isn't agreeing that whatever she says is right (that's just appeasement), trying to make her happy, never saying no, etc.
It's about figuring out the feeling behind her words, and putting yourself in her shoes. If you felt like your W suddenly changed after 20 years, and was talking to you "differently", but you couldn't put your finger on what it was, how would you feel? Confused? Angry? Scared? Other?
It isn't about if the context is "correct" for feeling the feeling. It's all about the feeling on its own. Fortunately, you don't have to agree with her that you're "untrustworthy, a betrayer, a liar", etc. You can validate the feeling behind all that and see if that helps you two connect.
An example might be:
Her: "I can't trust you anymore. You changed overnight after 20 years. I think you've just been lying."
You: "Wow, that would feel unsettling to feel like you couldn't trust me."
You aren't agreeing that she's right to feel that way -- but you're saying that it would be unsettling to feel like you couldn't trust your spouse!
...
Last thought:
Any advice on how I can help her understand why I appeased for so long?
If BPD is in the mix, it might be more effective not to pursue "trying to make her understand", and more effective to pursue improving your own health and building your own toolset. "Trying to make her see" sounds like a solid brick wall to beat your head against -- the wall won't break but your head will hurt.
Doing new actions (pairing validation and boundaries) may be more constructive that attempting new
explanations.
...
And a quick question to end with -- how old are your kids?