I understand how you feel, but I understand how she feels too.
Dude, this PHD is her entire fucking life. It's hundreds of thousands of dollars of debt, it's years of work and schooling, it's her only chance. If she doesn't finish it and get that job as a professor, she will be so entirely screwed, you don't even know.
Yes, her supervisors are fucking her over, but she can't complain. They'll simply get rid of her, for someone that's "willing to do the work". If she asks for time off it will piss off her coworkers as well, who are likely in the same position, and they will ostracize her, when she needs those people.
And honestly, I'm amazed she even has the energy to talk to you and have sex instead of just collapsing when she gets home like the majority of people do. I think she really does care about you. I think she is trying to pay attention to and be with you, and yeah that time is small and shitty and unfulfillng, but that's all she can give.
It's a shitty situation all around, and if you can't do it, you just can't, no one blames you, but it's not her fault, dude, and there's not much she can do about it. It's not as simple as waltzing into her bosses office and demanding more time off for her life and you and he says "WELL OK THEN!" and everything's peachy-keens. It's not as simple as "choosing" you and you two ride off into the sunset together for ever and happy and spending all the time in the world together. Life's not like that.