I'm with your girlfriend on this one. For me, the problem isn't that the show is horrible. The show is actually brilliant and engages the person to feel passionate about every character as if the viewer were a part of that storyline.
I can't speak on your girlfriend's behalf but the reason why I also absolutely DESPISE Walter White is that in the end, he's an egotistical, narcissistic coward who is bitter that he sold his old company's share for a twiddling $5000 and watched it grow into a billion-dollar industry. His vanity and pride takes him as far as killing other people (children and adults) to get what he wants and he shows absolutely no remorse for it. He plays the cancer card and lies profusely only when it benefits himself. He can't stand those who are superior to him, or even boss him around, and must be at the top, even if it means killing his way to the top. He's never satisfied with what he has (ex: he could've been out with $5 million but instead decided to keep going). He's a selfish prick who may have started on the meth cooking trail to save money for his family but ultimately he did it to fill a void surrounding his sad mediocre life ever since he left Gray Matter.
Another thing is that he keeps lying/making half-ass promises, such as "this is the last time a person will die because of us" sort of thing. Then bam, someone dies again. He's literally the grim reaper. He's also EXTREMELY manipulative to the point I want to punch the screen when his face is on it and throw it out the window (I don't do that cuz it's a Mac and it's my baby). Oh, and lastly, he makes that nauseating facial expression where he just lets his mouth hang open as if dumbfounded that he just couldn't believe the consequences that came with what shit has gone down due to his own selfish actions.
Despite my adamant hate towards Walter White, I give the writers full credit because they really succeeded at engaging my interest, attention, and most of all emotions into this character (even if it's negative).