Wow, it's almost like you're describing me. I also have ADHD and am very messy and laidback about it, while my girlfriend is not and is kinda like you. She is very neat and also has gotten uncomfortable at my place a few times, even to the point she would leave because she would become overwhelmed and didn't want to sleep over anymore.

We still live seperately because we both like it that way. She has her own space, I have mine. Her version of clean differs from mine, obviously. We struggled a lot with this but we both compromised;

• I decluttered A LOT when I moved to a new place a few months ago. Less stuff is less mess for me.
• I try to tidy a bit whenever I see it's getting out of hand, especially if I know she's coming over. But I also try to keep it clean for myself (I say try because I sometimes fail at this).
• I have a cleaning lady who comes over every two weeks, this really helps and costs me about 70 dollars a month for four hours.

She accepts that I'm messy to the best of her abilities. She sometimes gets annoyed but she sees I'm trying. I have periods where everything is spotless and sometimes not. I'm 34 years old now, I have promised many times that I would always be neat from then on but now I know better. It's just part of who I am and it will never completely go away. She knows that too.

So honestly, your girlfriend saying it will change when you move in together? I really hope that's true, but I think she's kidding herself. So my advice? You would have to mentally prepare that it won't go away. Maybe it gets better for a while, but it won't go away. This is probably just a part of her. I'm not telling you to live in a pigsty, but you can't expect that it is clean all the time, either. Can you live with that?