You have my sympathy too - my DD (nearly 18) is the same, and has A2 exams and AS resits starting in May.
I agree, nagging is counter-productive, but it is very difficult not to nag. I have, in the past, tried everything - nagging, not nagging, bribing, offering to help, buying revision guides and offering to go through them with her, talking about how this is only a few months out of her life, how it will help her have choices in the future etc., all to no avail.
In the end, for the sake of my relationship with her and for the rest of the family, I have decided to back off and let her make her own decisions. It was driving me mad. What has ultimately helped me is the realisation that her results and the course/job she then does as an adult, will have no impact on my life or my career. That has given me some perspective and peace.
It is the old cliché - you can lead a horse to water.... I have pushed my horse up to the water and stood over her at the water's edge, but she still won't drink.
It is so exasperating though, I completely understand.