Something has gone very wrong somewhere OP and now you are caught in an abusive cycle. I do wonder about the vagueness in that she left to stay with her GP's when she was 17.
TBH - sounds like you are saying and doing the right things. But even though she is your DD, she has no right to emotionally abuse you in this way. This is anger - pure and simple. She is in a rage and is lashing out.
Create the boundary and then enforce it. Comms by post sounds a good alternative. Letting her know you love her and will always be there. But equally not tolerating the verbal abuse.
My own background was utterly horrendous. Psychologically and emotionally abusive by both parents. I went NC with parents in September over their appalling treatment of DB and their behaviour towards me now as an adult.
Abusive parents don't stop just because their kids grows up.
Trouble is, I have children and have refused them access because they are very poor role models. They are now stalking, harassing me and hacking my social media sites and even work inbox. Police are involved.
I think one of the problems is that it took an amount of maturity to see my own background was abusive. Your DD is feeling the anger but probably isn't really sure why. She just feels there is something wrong.
Yes she needs help but that has to come from her.