Oh, I should mention, when she moved a few months back (after I’d ceased contacting her), she texted me at 6am on a Saturday morning to ask/demand that I be at her house to help her move by 9am.
I was honestly pretty stunned as she hadn’t been so demanding before - at least in terms of being clearly unreasonable and entitled - but in any case I didn’t respond and I was pretty peeved to be woken up by this.
I think this is a no-brainer. This isn’t your friend any longer.
Folk like the OP describes (I have run across a few of these myself) do need people, its just that they tend to need someone to enable their drama and reassure them that its the rest of the world that is fucked up not them. They don’t want to participate in your world as it might expose “the crazy” by contrast.
Normally I’d vote “No, but send a congrats,” but I read the whole OP before voting for once in my damn life, and I’m comfortable with “Ignore/Do not respond.” She’s been insanely rude before, multiple times, so you don’t owe her politeness or consideration in return. Any contact you make (even just a “Congrats! kthxbye” text) will suck you back into her dramaqueen act. OP, it sounds like you care about this person way, way, waaaaaaayyyyy more than she cares about you.
Dunno. Is it open bar?
Pravnik has the right answer.
Of course, an invitation 3 days before the wedding, which is on Christmas Eve is one I would feel entirely comfortable ignoring, regardless of the ins and outs of the relationship. Seriously, I spend major holidays with my family, I’m not dropping everything on a moments notice
This, down to the letter. (And the “help me move – NOW!” text sent at dawn is nuttier than squirrel turds. Yikes.)
By the sounds of things, she doesn’t want a friend, she wants a servant.
She sent you the invitation by text? I’d also add, “I thought I blocked you a long time ago!” to that. 
The question for me is whether you want her to know you don’t want to attend. You can not respond to a text, as there’s no proof you actually got it. However, you may want to be clear that you will not be attending, for which any of the polite responses above will do.
I do not believe that a single response via text would be sucking you back into her world. Heck, you could block her number after sending if you’re that worried you won’t have the willpower to stay away. I do it all the time. There’s no shame in knowing your limitations and taking proactive measures.
Also note that I’m often the type who says you should give someone another chance, but, by saying that your attendance is “important”, she’s doing the same thing she did before. And the only way to get through to her would be not to rise to the bait. (You don’t sound like you could handle actually just telling her what about her behavior is objectionable, and, even if you were, this is a lousy time to do it, so close to Christmas and a wedding.)
Oh, bullshit. When it’s really important to you that someone be at your fucking wedding, you give them more than three days notice. Particularly when you plan it for a date when most people got shit going on. Assuming, of course, it ever enters your pointy head that other people have lives outside of you, which I’m not sure is the case here. Send her a text congratulating her and saying you can’t make it because you already have family Christmas stuff going on, and then ignore anything else she has to say on the subject.
I used to have an emotional barnacle like this. She wasn’t quite so…entitled as yours, but about equally willing to follow reasonable boundaries and about as respectful of other people’s wants, needs, and priorities. Giving them the brush-off makes you feel like you’re kicking a puppy, one that’s mangy and wormy and sitting in a cold rain, because their behavior makes them so isolated, but at the same time, you don’t want to spend any time with them either for the same reason nobody else does.
The only thing you can do is this: If you’re going to engage with this person, do it entirely on your own terms, when and if you have the mental/emotional resources to waste on them. It’s not callousness or cruelty to ignore them except when it suits you, it’s self-preservation.
For what it’s worth, she didn’t just invite me today. She invited me last week, but frankly I forgot all about her text since I was busy at the time, then remembered I should probably respond with… something. So it’s not a three day notice, but it’s still pretty damn last minute considering it’s a Christmas Eve wedding.
I’ll note though, I went to her last wedding (she called it a commitment ceremony at the time, I’m pretty sure it’s the same deal) and it was just an in-church thing, no reception. A pretty informal affair all in all. It’s pretty much just a church service, but with a small (5-10 minute) ceremony in the middle. It’s not the kind of affair I would expect an engraved invitation to, though I think she texted me last time too, and I thought it was weird. But that was before the freakout happened.
I ended up sending her a polite private message in Facebook, as her wall is covered with a ton of updates about the wedding, of course.
Re invitations: Miss Manners says when someone invites you to something and you don’t want to go, just say, “I’m sorry. It’s impossible.” And give NO OTHER reason, explanation, or excuse. Just keep saying, “I’m sorry. It’s impossible” over and over until they stop asking you.
It’s not compassion to act like a friend to someone you don’t want to be friends with. You don’t have to “kick her to the curb,” but you also don’t have to ever see her or speak to her again. Just ignore her voicemails and texts.
In the old days, people had butlers to say, “Madam is not at home,” when everyone knew you were in the house. It meant “not at home TO YOU.” Now we have techie ways of accomplishing the same thing.
I also like, “I have plans.” My plan might be staying at home and not doing what you think I should be doing, but it is indeed my plan.
Doh! I meant, “No, I wouldn’t go, but I’d send a thank you for the invite, and congratulate her.” NOT “Yes, I’d go, but I’d resume avoiding her afterward.” :smack:
Miss Manners is also very passive aggressive. I wish she wouldn’t recommend doing so many things that expect the other person to “get the hint.” If you don’t want to talk to someone ever again, I wish mannerly thing was to tell them that, so they didn’t have to wonder.
I also have never understood your definition of compassion. Compassion requires doing something you don’t want to do. If you do it because you want to, then you’re doing it for yourself.