I can completely relate to you on this. I was diagnosed with ADHD as an adult some years ago and I too found a medication that worked. However, there are those days (sometimes multiple in a row) where my productivity level falls off a cliff. I can try every trick in the book, but the ability to focus just escapes me. It’s a horrible experience since I work in a job that is very demanding.
The part that frustrates me the most is that I cannot predict when this productivity “drought” will hit. And, for the sake of my job, I often find myself hoping and wishing that those days land on a weekend so it would not get me into trouble at work. When they do hit, however, I still find myself fluctuate between just accepting it and trying to fight it.