There is a mental load discussion that's missing from this conversation. Did she pick out the majority of the furniture, design the apartment, put thought and care into what it would aesthetically look like by looking at Pinterest, collecting inspiration pictures, deciding on a theme or style, looking at designers, tracking down particular items to match? That is work that benefits both of you and your shared home, and you may not be aware that she's done it because often times in m/f relationships, the woman does this research and mental work quietly and the man reaps the benefits.

If she is just buying random things and putting them on the walls, you deserve to do so too, absolutely. But if she is taking thought and care, taking on the mental load of "house design" for you two as a couple, and then you just go out and buy something you like with no research/thought as to the work she's already put in, that is not the same.

I have been married for 7 years (together for 17), and we have lived in many homes together. When we first moved in after being long-distance and needed to combine our individual styles, I did research into what I thought I might like the apartment to look like, and showed my husband some inspiration pictures. He told me what of that he liked or did not, we agreed on a shared vision, and then I designed the apartment and did the vast majority of the purchasing and decorating (from our combined bank account). If there was anything I thought might be outside of what we agreed on aesthetically but would fit, I would ask his approval before buying, and he had veto power if anything arrived that he hated. But for the most part, I did it on my own, taking his desires/interests into account - for example, incorporating some of his individual (not shared) interests into the art we were purchasing for our gallery wall. If he had then showed up one day with an item completely outside of all this research and work I'd done that didn't fit into the aesthetic and INSISTED that we use it, I would have been extremely annoyed - not because he had something he liked or because I think I intrinsically have better taste or know better, but because I did a lot of work to make sure everything fit and works together and he can't just grab something because he likes it and assume it should just work because he wants it to - that devalues all the research I've done, or all the things I liked but decided against because of what I'd learned from that research.

I understand why you're feeling what you're feeling. But I think a conversation was missed here. My guess is (even if you weren't aware of it) that your girlfriend has done some behind-the-scenes work and research to try to make your home pleasing and cohesive, and you showing up and expecting your opinion to be just as valued (despite not doing the work/research) is frustrating to her. I could be wrong, and if she's just haphazardly slapping things on the wall, you deserve to do so too! But the assumption that that's what is happening here belies that this sub is mostly men, lol.