my parents divorced when i was 3 so i've never really known them together. if the reason they're divorcing is due to something your dad has done.. it's understandable that she's got a lot of sh*t going on in her head. when you're upset you tend to hurt the ones you love the most. she may be suffering from depression. you could do with getting her to visit her doctor, it may help.
as for your a-levels.. don't give up. revision will make all the difference. you could always explain what's going on at home to your tutor to see if they could help you. they're meant to be there for you (but i wouldn't go to mine if i was in trouble if you paid me)
and as for your gf.. she obviously can't see how much everything else is screwing you up and so doesn't realise that her input in all this is messing with your head to. talk to her, explain why you feel the way you do, if she doesn't understand then she doesn't deserve you. without her as a distraction you may be able to concentrate on your a-levels more.