Maybe it’s just meant to be this way. It seems pretty stupid, but maybe me trying to figure all of this out is pretty pointless.
figuring it all out will aid you greatly in recovery (assuming the worst). understanding it from her perspective, as well as yours, as well as that of a more objective outsider, in the long run, takes the sting out, it puts the "coulda woulda shouldas" to rest (because its never about one thing), and there will be lessons you can carry into the next relationship that will make a better relationship more possible, and you an even better partner. finding that balance, for me, was closure.
its probably a little soon to be doing that, even if you wanted to in the moment. but when a relationship fails, its a failure to connect, or to resolve conflict. it isnt (usually) one persons fault, and some conflicts just cant be resolved.
How do I validate her in times like this? Is it even possible?
validation isnt about finding magic words to soothe someone. start from that premise.
one of the early concepts i was introduced to that helped me get it was the notion that "feelings are always valid". feelings may be extreme/disproportionate. conclusions about the feelings (not the same as the feelings themselves) may be extreme or flat out wrong.
and people with bpd traits feel everything on an extreme level. they are also easily prone to feeling invalidated (primarily the result of many, many insecurities). they are highly sensitive people (you and i may be highly sensitive people, if not bpd). and there are a lot of things, fears, insecurities, that tend to drive those feelings to extreme levels. underneath it all, the underlying feeling is generally valid.
validation is about first accepting that someone feels the way they feel, and then empathizing with it; putting yourself in their shoes and getting where they are coming from, and why they feel the way they feel.
begin by accepting that "throwing her out" (for lack of a better term) really hurt her. it felt like a huge betrayal. it felt personal. it felt like being kicked out by you, like being evicted by you. it hurt her enough that it has stuck with her for a very long time. remember, this is someone with lifelong, above average fears of abandonment. it further hurt her that you didnt call to check up to make sure she got home, a double whammy. virtually all women, generally more than men, look for signs that a relationship may not last. this was one such sign for her.
now, i dont know all the details about that night, but i gather she was being pretty ridiculous and hurtful, and i understand why you did it. in fact, i kicked my ex out, or left, or tried to do either one of those things, many, many times. i understand your side of it.
telling her your side of it is not validating. it may even be invalidating, at least in the moment. thats not to say there isnt a time or place to tell your side of it. the question, when dealing with a highly sensitive person, is how, and when, and when they (anyone) are worked up and most wanting to be heard is often the worst time. loving someone with bpd is generally about learning to be an even better listener.
its also not to say she wasnt totally unreasonable about it. the fact is, she was so hurt by it, that she couldnt see beyond that hurt (weve all been there. think about a time that you were. people with bpd traits are like that, about the big stuff, and what seems to us, like the small stuff). maybe, at times, she could, but ultimately, she couldnt.
ill give you an example. my ex and i fought a lot about me looking at porn. its a long story, full of attempts on both our parts to resolve the issue, full of hours long fights, full of each of us explaining, time and time again, our positions on it, it includes broken promises (on both sides), it even includes the fun time she found (the next morning) that i had looked the previous night, and her screaming bloody murder outside of my home, very early in the morning, causing my neighbors to come outside. good times.
eventually, a year or so before we broke up, i told her i was done with the issue, done fighting about it, and that i was going to look. that if that was a significant enough problem, that i understood if she couldnt continue the relationship (she mostly just heard this as me saying porn was more important than her. it wasnt, to me, but i can understand exactly why she felt that way). some time before we did break up, she told me how much that broke her, hurt her, and caused her lingering resentment toward me. i can understand that. i dont agree with all of her conclusions (and i can tell you my own very different side, i can tell you about all of her snooping, and i can tell you about her numerous double standards), but it pained me to know that it had done that to her.
it was one of a number of conflicts we couldnt resolve, and things we couldnt accept about each other; not necessarily the particular one, but it was a big contributor. it was something she couldnt get past. and i think her feelings about it were absolutely valid. its not a dissimilar example, really. a lot of us have these stories about things our loved ones (past or present) couldnt/cant get past. the fact that some remain together in spite of it tells you that a breakup is usually about more than one "thing".
as for her insecurity about your ex wife? thats tough. people with bpd traits, some more than others, can be especially jealous, possessive, insecure, what have you. mine was, and mine actually accused me of the same thing with an ex, which was preposterous. there are ways of dealing with a jealous person. it doesnt mean you ever make the jealousy go away, or that she would never have been insecure about your ex wife, and continued to bring it up. accomplishing that was/is pretty unlikely. validating also does not mean agreeing; you dont want to agree with or fuel what is a preposterous perception.
validating in that case, is again, about understanding the underlying feeling thats driving it. maybe she didnt feel prioritized by you. maybe she felt insignificant or second place. this happens all the time in sort of blended families, or when trying to blend families. its hard to say, and you would likely have better insight on how she felt (in retrospect, between the lines and conclusions and hyperbole, she probably told you more clearly than it might seem), but any of those things are valid feelings any of us might experience.
validating is about finding the underlying feeling, accepting it, getting it, and demonstrating that. what you do, or say, beyond that, depends.
it is completely unrealistic, by the way, that you would never hear from her again.