You're getting good advice in this thread. Let me add what I hope is a little more.
For starters, you have to understand that any mentally or emotionally ill person is, by definition, self-centered. I don't mean that necessarily in an evil or villainous way. I mean that their needs take the primary focus in their lives. They are driven by a universe of needs. They exist for them.
Someone with BPD (or anxiety, CPTSD, etc.) is even more so because invariably they suck other people into their confused and malignant universe. Some do it less than others -- the so-called quiet ones. Some realize the depth of their illness and withdraw. But even they pull someone into their orbit eventually. It's the nature of their illness.
As others have pointed out, trying to find a rational explanation for their behavior is next to impossible. Their behavior may or may not make sense to them, but that doesn't matter because -- and this is the important part -- none of that stops them. They act first, think later, even when they plot. They don't stop to ask if they should stop, at least not in a way meaningful enough they might actually do it. Or not do it.
So, when your ex calls or texts you out of the blue, it's an irresistible impulse, not a rational choice. If you think of it in any other way, you'll only suffer. You'll think it's because they love you or care about you or are just thinking about you in a way that a healthy person does. It's not. It's the irresistible impulse.
That's not to say they can't love or care about you, to the limited degree they're capable. There is lots of debate about that, but I think it's pretty clear they're too consumed by their own self-centeredness to really love or care enough about anyone else to control their irresistible impulses. But you see, that's the trap. A rational person who can control themselves always imagines the other person can, too, and with someone with BPD (or anxiety, CPTSD, etc.), that's just not true.
So, you must not think of the call or text as a kind of lifeline tossed out to you by someone who is struggling to be healthy and only wants the best for you. It's not. They're not. They're simply caught up in a moment that will change to yet another in the blink of an eye. It's not in their control, and it's not in yours. Your best bet is to stay NC and stay sane.


