I think you're doing the right thing. Honestly, it sounds like your daughter has a case of, "I'm an ADULT! I don't have to abide by your stupid rules!... But you should still care of all of the little details for me!"
One of my mother's coworkers, Stephanie, could use a dose of your fortitude. Her oldest daughter, Becky, has some mental health issues, which have allowed the daughter to manipulate the parents out of holding any boundaries with her or enforce any plans for completing educational/treatment programs. Stephanie and her husband try to draw a hard line with her, Becky threatens self-harm, they back off. And when her therapists recommend solutions, Stephanie, refuses to follow their advice, stating that the solution won't work for some reason. So they remain in an unhealthy cycle. Becky lives at home, doesn't have a job, doesn't go to school, but has plenty of time to shop online (with mom and dad's credit cards), and drive mom and dad's cars, since she totaled the one her parents bought her. ETA: Even my mom, whose coddling of my baby brother, Ted, is the stuff of Ehell legend, says, "Wow, that's messed up."
What's worse, is there's a younger daughter in the house, Jenny, who is more functional and a good student. But Becky latched onto Jenny, claiming that she couldn't cope unless Jenny is sitting at home with her, keeping her company. (Imagine a "companion" that a pampered daughter might enjoy in a Jane Austen novel. Becky sits around on the couch, watching TV, Jenny sits there watching Becky watch TV. If Jenny tries to read or work on her laptop or direct her attention elsewhere, Becky pitches a fit.)
I suspect that Becky is jealous that Jenny has lots of friends and enjoyed school and was trying to keep her from enjoying her senior year. Jenny missed out on a lot of socializing in her last year of high school because "Becky needed her." Becky even tried to prevent Jenny from leaving for college this fall, because she wanted Jenny to stay home with her. And Stephanie actually asked Jenny to consider deferring for a year and going to a local community college until Becky "got back on her feet!"
Thank goodness Jenny refused and the dad backed her. Because Becky is never going to "get back on her feet" as long as she's allowed to do what she pleases. I don't even think Stephanie really wanted Jenny to stay home from college, but she's so used to appeasing Becky that she can't pull out of the cycle.
So really, you're doing the right thing. Your daughter needs to understand how the world works. She can't sneer at you and your stupid rules, then beg mommy to fix everything for her. Stay strong.