I could give the whole story but here's the bottom line. My home is a happier place when she's not in it. Inserting her personality into a crisis is like pouring gasoline on a dumpster fire.
Raliced is right... 21 is the age of blaming parents. Very similar to MLCers blaming LBS for all their woes.
I use my ninja DBing skills to deal with my boomerang S23. I also use Al-Anon techniques. My S has been much more supportive than your D21 has been, no gasoline on a fire, not at all. But he's not the best roomie, he makes terrible life decisions (didn't respond right away to a job offer for excellent pay and lost it, etc), and I really think he should be on his own by now.
I do not ask him what he is doing with his life, or if he applied for any better-paying jobs, or if he paid that parking ticket he got last week, etc., because that is none of my business. (Thank you, AL-ANON and Byron Katie.) No ultimatums, no unsolicited advice.
These things, however, ARE my business:
1) Is he a good renter? (I charge him about half of what it would cost to rent a room in our town). Does he abuse our property or does he respect it?
2) Is he a good roommate? Does he abuse his roommate and our neighbors or does he respect them?
Your D21 sounds like she is not a good roommate.
When my S broke agreements, was messy, didn't pay rent, etc., I told him he was not being a good roommate and I thought he would be happier living with someone else. I asked him what he needed to do to find another living arrangement and move out. He started working on it, complaining vociferously! Ultimately he decided to be a better roommate.
Our lives are much smoother than they were. Sometimes he asks for advice. I give it but then tell him I have every confidence he will figure it out on his own, and then I let it go. That's hard to do and sometimes I mess up. No biggie. Just as long as I get on track again.
Sometimes he just wants to tell me something without me offering advice. Also hard for me to do.
M:54, H:55 T:33, M:27 12/13 BD: EA 01/14 BD: PA, H leaves 03/14 H & OW break up 05/14 H says he will file for D 08/14 H initiates D 09/14 H wants to R 12/14 Still bungling our way through R