What I'm about to offer, qazmatoz is basic Psychology / Human Sexuality 101 stuff, so it may be obvious to some of you guys.[footnote]In my not-so-humble opinion, this stuff should be obvious to everyone, but for our crappy educational system, and the US's ongoing freak-out about sex.[/footnote]
All that crap your girlfriend is dumping on you is not about you. The problem here is that she's insecure about herself, and is seeking out verification of her own self-image (her waning sex appeal) in your performance difficulties. The fact is, even if you were performing like a pro, she'd be on the lookout for reasons she is on the out. Note that this has nothing to do with the truth of the matter. She may still be a hottie (and tell her so!) but she will ultimately need to decide for herself that she's attractive enough (or that it doesn't matter).
The ideal solution is to take off the pressure to perform. As Dan Savage likes to point out, the more different activities you considered as having sex, the more sex you will have.[footnote]This is a detail that heterosexual groups take for granted, since coitus is usually an option. In the gay communities, many couples love each other, and want to express that physically, but don't exactly have matching equipment, so for them experimentation is a natural process.[/footnote] Maybe your girlfriend would be willing to take copulation off the table for a while, while you guys just experiment and explore and find out what all feels good. If a good time doesn't require the necessity of an erection, or of successful insertion, that will ease the pressure to make such things happen.
As was noted above, there's always the All-American way of prescription drugs. The Viagra-style meds that are out there are remarkably effective, and are used for a myriad of causes for erectile dysfunction, from simple condom resistance to prostate failure to countering the side-effects of antidepressants. I don't recommend going this route, as this only treats the symptoms of the larger issues. Communication problems and stress will manifest even worse symptoms with time until they are addressed.
Unlike my fellow Escapists above, I do not recommend curbing your porn habit unless it is particularly extreme (say you're masturbating multiple times a day) in which case the issue isn't specifically porn but manic excess. But masturbation is sex with yourself, and is inappropriately disparaged thanks to religious and cultural issues. It is good to know how your girlfriend reacts to you having porn, and seeing what porn you have. If she doesn't like it, you need to exercise discretion, of course. Asking you to not have any, though, is unreasonable; even Osama Bin Laden had a porn stash.
If it's not obvious, people in porn, like people on television, are not built like real people, and it is good to be aware of how your brain reacts to one compared to the other. Western culture has had problems with young men being exposed only to playboy centerfolds and imprinting on them rather than real women, but this is something usually that plagues inexperienced single men; I suspect you don't have this issue.
Good luck.
238U