DXMKid, I can relate to what you're going through as it happened to me a while ago; my DXM usage unfortunately coincided with a period when I was in a serious relationship with a girl several months in - she is particularly anti-drug - and let me tell you that I can sort of fathom why your girl would lash out at you, especially considering what you said about being constantly high on dex.
You're telling us you've been on a constant DXM afterglow/plateau day after day after day? You are aware of how someone appears in such a haze right? My girl would get so pissed and teary when she saw me sweating or bug-eyed or slurring, she'd just have this pained look and spout similar shit. Put yourself in her shoes; how would YOU feel meeting your SO for an anniversary date and they have Powerpuff Girl eyes, completely out of whack emotionally and the verbal skills of a 10 year old with Down syndrome?
Your girlfriend is obviously just worried about what's happening to the person she cares deeply for. She's worried you may not be the same person she once fell for/knew, that you'll transform into some horrid shell of who you once were. She herself must be feeling some guilt, feeling powerless at the fact that she feels there is nothing she can do to help you.
Take this to heart:
Everything in moderation, even moderation. It will be hard but you've got to CHILL with the constant dexing. Taper off it slowly, limit it. If you truly do not want to lose this girl then make her your priority, not cough syrup. Sit and have a talk with her. I'm fortunate enough to have come to my senses and still am with my girl to this day despite my overindulgent stint with DXM (though I do indulge from time to time on my freer of days :D)
You may not be able to quit, but goddamnit man dose smartly. Drugs shouldn't be at the absolute forefront of your life, overshadowing everything else. You're just setting yourself up for failure and disappointment somewhere down the line. Drugs are something we all like to do (hey, this is BL

), but it shouldn't be the ONLY thing to do.
Especially if it's starting to negatively affect other aspects of your life, is a good sign that you need to chill.
You can do it man. Cough syrup can always wait. CVS/Walgreens/Rite Aid/Duane Reade are still going to exist, and will always have cough and cold aisles. Your girlfriend, on the other hand, won't. Your girlfriend will always care for you. Robitussin doesn't give a shit about you.
Re-evaluate
everything.