Hi there jumpingtothunder,
It kind of sounds like she smothers you. Like it's "all, or nothing".
On the other hand, with your common interests, you've also had good times together, and when it's been good, it's felt good.
However, it also sounds like she can't respect boundaries. For a healthy person who hears their friend needs some space during a health crisis, they would be able to take a step back and say, "ok, when you are ready, let me know how I can help". But instead of that, she isn't able to hear that you are unavailable to her, since you are overwhelmed with the health issues and just need your own space. This is her problem, and you need to give her time to soothe herself. Trust me, she will. I have watched this pattern in my uBPD mom her whole life. My mom's MO is "an eye for an eye and a tooth for a tooth", when some does or says something she doesn't like. It could take a week, a month, a year. But eventually she will come back to you when her need to have you in her life supercedes her need for revenge. There is likely nothing you can do or say to mollify her, and the more you try, the more power you give her to control your own feelings about it all. That will just feed the dysfunction. Instead, let her have her feelings for as long as she needs to. When she is ready, she will call you up one day, and pretend nothing ever happened, which is also going to feel bizarre.
About a week later, she posted a Facebook status about a week after we had talked saying she’s “less likely to do distanced communication as the weather warms up.” Since I initiated our last conversation to clear things up, she hasn’t called or texted me once.
This is exactly what I'm talking about (eye for eye and tooth for tooth). Just let her be, and give her time to self-regulate in her own way. If,when you have contact through family functions, just be cordial, follow her lead. If she ignores you, go along with it and let it be. If she is friendly, be friendly. Do not make a reference to the past. Do not bring it up. Do not apologize. Now is the time to set healthy patterns for the future. Don't play or feed into any dysfunction. Remind yourself that there is nothing to "clear up", because you did nothing wrong.
I feel like it’s my fault for either not being so specific about the timeframe for which I needed space, but also because I know (from her and I talking) that it caused her to feel devastated, which involved my BIL talking her through how different people need different things.
This is NOT your fault. You can't fix this. You can't fix her. Let her be, and self soothe. She will. Give her time. If you had given her a time frame (how could you unless you have a crystal ball and know the future?), she would have found some other reason to project her bad feelings onto you.
I’ve been stewing in my own anxiety about it for weeks
This is just a huge misplacement of energy right? By stewing over it like this, you are giving her control of your feelings. That is not ok. You were completely within your rights to ask for some space from a person who doesn't have the emotional skills to recognize that she was smothering you, and causing you more stress rather than less. Again let her have her own feelings. Don't try to fix them for her. You can't. Her feelings and your feelings should be differentiated, because you are two different people; you are not the same person. When she's feeling bad, she should not be making you responsible for that. Healthy people don't do that.
I just don’t know how to shake this resurfaced trauma of feeling like even if I do care or have expressed care in the past, it’s not enough especially when I need space.
Radical acceptance of my mom's uBPD helped me. Once I accepted her as she is, and "met her where she's at", things got better. There's still ups and downs when she dysregulates, but I'm better able to navigate those times now. Whatever you do,
don't JADE. The best thing you can do is let her self-soothe. When she does, she's going to pretend nothing ever happened, and you will know when that time comes.