@bendmeoverbackwards you've been really brave to be so honest about how things are. You can't change everything at once, so pick one thing.
It sounds like the way she communicates is the priority because it's what's she's using to check that she's in control of her life. I would say the following by message, so that she can read it and re-read it, and can't change what was said later:
"DD3, I'm sorry that you've felt so let down by your teenage years and that you blame us for your current situation. You've told us about it so many times, and we understand how you feel, and that we can't change it or undo it by apologising.
The way we communicate isn't working for any of us. You don't feel heard, and nothing we say satisfies you. It has to change.
Going forward, the following changes are going to be made:
- we will not be responding to messages that discuss the past. If you want to talk things through, we can do that but it has to be at a time that is convenient for both of us.
- text messages are only appropriate for factual questions or information. For example 'will you be in at 6pm?'
- we will not restrict the way we communicate with you. If Dad wants to tell you something or ask you something, he will do so. I can't be the only one who talks to you.
- I need you to stop refusing to do things because I'm not nice enough. I can't take responsibility for your decisions.
- I can't deal with late night conversations. I won't discuss emotive issues after 9.30pm. I'm willing to talk but I can't sleep if I am talking about emotional things late at night.
- Our bedroom is off limits. If I have gone to bed, or haven't got up yet, you'll have to wait until I'm downstairs.
We love you and we're committed to you, but it isn't helping you to allow you to blame us for decisions you are making. If you want to talk about how you can make small steps of progress, that would be brilliant. But we're not going to keep going over the past."
Then stick to it:
- If she tries to discuss something by text: "Remember we don't do text messages for this stuff now. Come and talk to me/I can talk at 4pm/we can talk tomorrow"
- If she comes in to your room: "DD2, our room is off-limits. We can talk later/tomorrow." If she continues, ignore her. She will run out of steam eventually. It will take ages the first time, but it will take less and less time if you stick to it.
- If she's blaming you 'Sorry you feel that way.' then ignore.
- If she kicks off about DH 'DF has told you what's happening' then ignore.