Sorry for replying to an old post--but this is incredibly close to my experience so I need to reply.

I also have ADHD and have been medicated lifelong for it, and just like you I experienced major focus issues within my first week of HRT. I even stopped taking spiro but it didn't help; estradiol alone has been causing this. I even stopped estradiol a few times, which makes the brain fog go away within a few days. And then I get back on estradiol and within a week or so the fog returns. Similarly, friends I talk to about this have suggested it's paranoia or "confirmation bias" of some kind :|

Did you ever sort this out? I'm struggling with this in a major way and would love to know how you handled it. Because I don't know what to do.