Goodness, I never thought about people not really existing nellie. Guess that’s me being naive and not an ADD trait! 
Best thing you can do William is to point your girlfriend in the direction of some of the posts on this forum. I guess it’s difficult for a non-ADDer to understand. Imagine a so called ‘normal’ looking at my life for instance. I would just appear as an irritable lazy slob. There’s no reason for ‘them’ to understand there’s a block stopping me from doing what should be done (assuming I even notice what should be done!), OR why I have so many half started and incomplete tasks.
It’s hard not to dwell on it once you’ve go a diagnosis, but then you can move forward hopefully.
I have only told one close friend about it. I jokingly said in my defence at work (I hadn’t done something despite numerous reminders) that I couldn’t remember because I had an attention problem. The response was a groan and “you’re not hiding behind that excuse are you”. So I’m guessing unless you have it, there will be little understanding from that linear section of society.
So William, she is not right, she doesn’t understand ADHD. However, you do understand it hopefully or are starting to understand it in your own context, so you can perhaps modify things that annoy her. On the other hand, it’s a mistake thinking someone will change as a relationship develops, so maybe you need to sit down and discuss things with her as no-dopamine suggests.
Of course I am hopeless with relationships so best not to take my advice! My initial thought was to tell her to take a hike (out of your life) but the meds kicked in during writing this and they must work because that wasn’t my first suggestion!! 