I think there is some middle ground between backing off completely and continuing as you are. I think the thing to do–if you can–is to stay involved on a practical level, but stop being involved on an emotional level. It’s the emotional level that is wearing you out and that is turning this into a power struggle.
Sit down and decide what you think an acceptable level of perfprmance is. Then, come up with an incentive you are willing to give if that perfprmance level is reached, and a ppunnishment you can stand to administer if that performance level is not reached.
The performance level you decide on needs to be objective and reasonable. Do not say "If your grades don’t come up. . . ", say “You need to have all Bs. For every A, you can have a C.”
Picking the right punishment is the hardest thing: it has to be something that is severe enough to matter, but not so draconian that you won’t be able to carry thrugh with it: cutting off all contact with friends and removing the TV and the internet and making her wear corduroy toughskins to school every day for the next 9 weeks is ridiculous and counterproductive. Six weeks into it she’ll hate you and you will hate you and you will cave. You might present a list of privledges: Phone, Internet, Going out Friday, Going out Saturday, Friends over after school, TV on Weekdays, TV on Weekends, and then say that for every grade that is below a B, she loses one of those things until the next grading period. Hell, let her chose which thing, when the time comes.
Another important thing is not to land on a punnishment that requires more work on your part than you re willing to do: don’t say that if her grades come down you are going to call her teachers for conference every week unless you are the sort of person who is good at making seven calls a week without fail. Don’t suspend all driving privledges and then make exceptions for running your errands.
As far as rewards go, you have a better idea what she wants in life than any of us: come up with something suitably juicy, but not ridiculous, and I think that things are better than money: money is crass. Better to pay for her to get a manicure, or give tickets to a concert or something like that.
Once you have set up this system–and you will have to get your X on board, though all she will have to do is monitor any privledge losses that come about–and you may want to stick to privledge losses that you can do all the monitoring over–explain it to your daughter and step back. Don’t harp on it, don’t worry about it. Once a week, ask her if there is anything going on with her schoolwork that you could help her with. Then, when report cards come, see what’s what. If her grades are low, don’t take it personally. This is the hardest part. Don’t be angry. Don’t be disappointed. Have a matter of fact “That really sucks for you. Which privledges are you going to lose?” attitude. Do be mildly encouraging: “Ah, well, nine weeks isn’t so long. I’m sure you can pull up English and Chem for next time.” Do not accept excuses, no matter how valid: tell her that if she was having problems, the time to bring them up was before the end of the grading period. Again, don’t be emotional about any of this.