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I CANNOT be the only one who thinks that sex should be about pleasure and not about performance. I mean, yeah, it's admirable that you want to perform well but... hell, that's not what sex is about and you have to think of yourself just as much as you have to think of her.
If you fuck not for fun but because of a twisted sense of duty, then of course you won't get it up. I had a relationship like that and it was the beginning of a horrible ending.
The hottest sex I had was with a girl who knew how to please herself. This may sound weird but you really DO need two to tango and if she really is that thick that she thinks YOU have to work for HER pleasure then it's her own fault. This is not the macho in me talking but I honestly believe that sexual emancipation means that you have to search for your own pleasure, not lay on the bed like a dead fish waiting for a dicking.
Now before you go breaking up with her, it sounds like she's really insecure which could mean that she's still not really opening up to you despite having sex with you. This insecurity will be even worse on the long run after you solved your stress problem at school. Work on that. Help her feel less insecure sexually. Watch her masturbate or even help her do so. Please her in other ways than through dickings (orally, manually, depending on your age and availability toys) and:
I suggest that you suggest her to actually take the initiative and try to please you. Tell her how she could 'perform' to make you go off despite any stress.
AND FOR CHRIST'S SAKE, HIDE ANY TRACES OF PORN. I know that EVERY guy watches it but you don't have to spell it out to her *facepalm*
tl;dr
Sex is NOT about performing, it's about fun. Find that and all stress and insecurity will be meaningless when you are intimate with someone.