My D took a semester off after freshman year. Her situation was different from your D’s in some respects, in that she was miserable the whole first year and wanted to transfer someplace else. But it was similar in others – perfectionism, and too many consecutive years of high-pressure academics (she’d been in IB since 6th grade). And she had struggled with depression, which I think is entwined with the perfectionism. At the end of freshman year, she applied for a personal leave of one year, and intended never to go back there. Spoiler alert:
She found she missed it more than she’d ever imagined she would, and went back after only a semester off, ended up loving it, and graduated with honors.
Like you, we worried that she’d never finish school, but we knew that in her state of mind, nothing good was going to come of forcing her to stay, and we could see that she really did need a break from school. So we reluctantly allowed her to apply for the leave. One thing we did which eased our minds just a little: Before she left, we required her to come up with a concrete plan as to how she was going to finish college, including what schools she could transfer to, and how it would be paid for since she’d be giving up a hefty scholarship if she left her school. She also had to give us a detailed plan about what she’d be doing during the time off. Your D’s situation isn’t nearly that dire, but you may want to have that conversation with her about her year off, and about what it will be like going back for senior year which will start making it concrete in her head that she really will go back.
But what helped the most was, as much as we could, letting go. Somewhere in that miserable spring of 2010, we made the decision in our own minds that this was her life, her education, her choice. Ultimately, we really had no control over what she’d do. We could only control us, and that included not alienating her from us by trying to force our wishes on her. So we tried to be supportive of D the person, as opposed to D the student. We constantly reminded ourselves (and occasionally told her) that she was an adult, that she knew the situation and the stakes and the difficulties, that she was capable of making this decision. We were always available for advice or as sounding boards, and we did encourage her to think things through. But we tried as best we could to back off, wayyyy off. Don’t get me wrong, we 100% wanted her to go back, but we decided that if it wasn’t meant to be, we had to be OK with whatever alternate path she chose. And we decided that if she did go back she – not we – should “own” it. We communicated that to her too, which (she told us much later) was tremendously helpful to her in trying to separate what SHE wanted from parental expectations. It helped us sleep better at night, understanding that it wasn’t up to us to control the outcome.
So my thoughts FWIW: Your D is a smart and accomplished young lady, and she understands the importance of a diploma. I completely get the academic exhaustion, which is not the same thing as rejecting education. Help her talk through the decision and try to tease out concrete plans. Support and encourage her. Be available as a sounding board. And then leave her to her own decision process. I think it’s highly likely that she’ll go back, but even if she doesn’t, she’s not going to end up waiting tables for life, She will take another path to wonderful things. I’d put money on it. 
I like @Python20 's suggestion of taking a fun easy elective. You may want to suggest that to her, or even a 101 class in a totally different field which has always intrigued her. If you can get her to browse the course catalog, even better, because it makes it more specific and more real.