I find the reason as irrational as the initial act, but she says it with such ferociousness and conviction, that nobody generally dares argues with her.
You seem to be describing a woman who experiences such intense feelings that she has great difficulty challenging them intellectually. Instead, she simply accepts the feeling as accurately reflecting reality. This "feelings are facts" view of life, if it is occurring, means you cannot reason with her rationally.
Significantly, this distorted view of other peoples' intentions (namely, YOUR intentions) is something you can readily identify with. It happens to you -- indeed, to all of us -- whenever we experience very intense feelings. When you get very angry or infatuated, for example, your judgment goes out the window. That's why, when you are very angry, you try to take no action and say nothing until you have time to cool down. And, that's why, when you were infatuted, you waited a long time before buying the ring.
There is always a "reason" for anything negative she says or does.
When a person is unable to regulate her emotions properly, her feelings become so intense that she is absolutely convinced they MUST be true. The logical part of her mind is therefore severely constrained. With her intuitive mind being fully in control, the logical part is given the silly task of coming up with a rational explanation for what she has already concluded must be true. And, if that shoe doesn't fit, she will quickly create another shoe, i.e., another rationalization.
Any blame on her is immediately redirected at me.
If she has a very weak self image, being "The Victim" is the closest thing she may have to a self image. If so, she will keep a death grip on that false self image by blaming every misfortune on you. As long as you are always "The Perpetrator," you will continually "validate" her role of being "The Victim."
Just read your post on BDP. I've having a think about whether it's her, me or both!
It's both. Because BPD is a spectrum disorder, we all are BPDers to some degree -- in the sense that we all occasionally exhibit all nine of the BPD traits. Such traits arise from our primitive ego defenses and, at low levels, they are essential to our survival.
They become a problem (i.e., become a "disorder") only when they are sufficiently strong to undermine our relationships by frequently distorting our perceptions of other peoples' intentions. Moreover, even healthy people can get flair-ups (from a hormone change, sudden trauma, head injury, or drugs) that are so strong that they temporarily behave like full-blown BPDers.
The fear is all directed at me.
If she has a strong fear of abandonment, that is exactly what you should expect. People fearing abandonment typically interact very well with casual friends, business associates, and total strangers. None of those folks pose a threat because there is no close relationship to be abandoned. They also do well with young children because the kids are so dependent on the parent that abandonment is not a real concern. When the kids get older, however, the fear usually extends to them too.
That said, Poet, I have the feeling I am going off-topic and not meeting your needs -- i.e., perhaps moving in the direction of "conclusions" and away from "temporary positives."