It sounds to me like you have two problems; mess, and clutter. Trash needs to be thrown away and dishes need to be done. That's mess, that's one issue. Then you've got clutter - items - clothes, makeup, shoes, magazines, electronics - whatever.
You're probably going to have to spend some money and do an overhaul of how your home is organized - is your home organized?
I've moved a lot. 6 times since 2009. Every time I move I go through my clothes and shoes and donate whatever doesn't fit, I no longer like, and throw away stuff that is worn out. Basically if I don't use it regularly, it's gone with few exceptions. Does she have a bunch of shit in her life she doesn't use? She needs to go through it and get rid of it.
I did the same with my makeup last year, threw out 1/3 of it. I did the same with my skin and other beauty products last month. I threw out probably half a trash bag of shit I'd been holding onto because I "might use it one day." No. If you don't use it, get rid of it. I also ridded myself of a bunch of old notebooks full of college notes, and soon I'm going to need to get rid of textbooks I know I'll never look at again but I haven't gotten around to it.
All of the people in my house when we check the mail, if there's a piece that isn't specifically addressed to one of us it goes in the bin outside before it even makes it into the house.
To keep our living room clean we bought some shelves, an entertainment center and a coffee table with drawers. We now have a place for all electronics, game controllers, and video games/ DVDs.
The last kitchen I lived in was a wreck because it had little storage space. The kitchen I have now, off the top of my head has at least 7 drawers and 15 cabinets. There is a place for everything (and when we combined households we got rid of dishes we didn't need.)
My boyfriend and I just bought new means of clothes storage and I bought bigger clothes baskets because our closets and floors were a wreck - but we freed up closet spaces and now all of our clothes have a place to go. I actually created so much storage in my room that I got to move all of my makeup and skincare into my bedroom, freeing up mess in the bathroom.
There are books about this. Downsizing the amount of shit you own and finding a place for all of it to go. It takes time and probably money but it is so doable and every time we work on this I love my home more and more - it is wonderful.
As for general mess, I've had to make some compromises. I cannot for the life of me get my boyfriend or our roommate to just do their dishes after they cook (we have no dish washer) so more or less I let the dishes pile up and watch them fight about who is going to do them once a week. I take care of my dishes when I use them so it simply isn't my problem. If I do leave my dishes and they've got a couple around, I'll do dishes for everyone (it's a nice thing to do) but this isn't often. We have no problem telling one another "you need to do your dishes." You need to be okay with telling your girlfriend that.
They leave food packaging/ used paper towels/ cans/ bottles in the kitchen (there's a fucking trashcan, how hard is it to use?) and I hate having trash around and it takes me five seconds to throw it away. I also take out the trash and recycling most often because they're happy (lazy) to let it pile up and overflow. The last time I pointed out that a trashcan exists and that when it gets full it needs to be taken out, my roommate accused me of being a passive aggressive asshole but id rather be that than act like a child. He still doesn't take out the trash, by the way, so it had no affect. Asking nicely has never worked and being mean didn't either so I don't really know what to do about it.
These things drive me fucking crazy but there's little I can do about it because they simply don't care, and whoever cares the least wins these battles, unfortunately. I'm also in school right now and I work less than they do so I don't feel as bad doing more cleaning, but I feel like once I get a full time job I might lose my fucking mind.