It's hard, and Finallyblooming had a lot of good points.
It all started a few months ago when she demanded to know why she and I do not have a close relationship
I've heard this a bunch too. What she's really saying to me is, "You used to do everything I wanted without question. Now you tell me no."
I've tried multiple times trying to sit down and talk about the issues in our relationship (all before I realized she has BPD traits), and I can say from experience that expecting them to understand your needs and meet you half-way are probably not going to happen. Everything that's wrong with the relationship is your fault, you're not trying, she's trying so hard to make things work, isn't what she's doing enough for you, etc.
It sounds like you were always the one calling her. You can't do all the work for the relationship by yourself.
I wrote an email to myself expressing what I want to tell my mother and left it at that.
This sounds like an excellent idea. That way you were able to still express your feelings without fueling the fire.
she half-apologized in the last email, which makes this one tough
A half-apology isn't a true apology. Did she find a way to still not take accountability about whatever the issue was? My mom is a pro at the blame-apology. "I'm sorry I can't give you what you want!" Then it's magically my fault that she failed.
Do I try to tell her the truth, knowing full well it will blow up in my face?
It sounds like you already know that she isn't capable of handling the truth, and you're just looking for some validation.
You're not crazy.
It's not you.
You are not alone.
A relationship is two people.
You can't make her want one.
You can't make her want help.
All you can do is protect you, and do what makes you happy and healthy.
It might help to figure out exactly what you want from the relationship. Then figure out if it's possible, and what you can do to get there. Setting boundaries are a great way to to both figure out what is important to you and keep other people from walking all over you. It's not easy, but there's plenty of people here who completely understand what you're going through.