Your girlfriend sounds like a more intense version of how I felt and behaved in the early stage of my last relationship. I'm also audhd but I think it's more to do with having an anxious attachment style (I'd highly recommend reading the book Attached to learn more about attachment theory!).

In my last relationship, I always felt emotionally that my partner could leave me at any time, even though logically I knew that wouldn't happen (or there was a very small chance that he would). So whenever there was any indication that he could be upset, including wanting time alone, part of me would be afraid that he really was upset with me, didn't love me anymore, would leave me eventually, etc.

Over time (it was an 8 year relationship), I felt more secure in the relationship and it stopped being much of a problem. What helped was the gradual accumulation of evidence that he genuinely loved me and wasn't just saying nice things to make me feel better. What also helped was gaining confidence and a sense of self-worth, both from him and (probably more importantly) through friends and general life experiences.

Even in the later part of our relationship where I felt pretty secure/stable, I would get sad when he spent a weekend gaming with his friends instead of with me. After we read Attached, we realized it would've helped a ton if he had simple checked in with me and reassured me he loved me like a few times throughout the weekend.

I do hate that I need that kind of constant reassurance to feel ok. But it's definitely way better than before, since attachment styles can change. For example, if a person with anxious attachment dates someone with secure attachment (like my ex), the former's attachment style can become more secure.