I know that feel man.
It happens to me a lot, so now I've resigned myself to it, I don't really pay attention anymore.
Instead of retaining the long sentences, I focus on the idea itself and end up with a kind of punchline, the substance of the sentence and the idea behind it. Three or four punchy words.
If I try to write a long, structured sentence, my mind goes faster than my hand and I end up with a blank page.
It played tricks on me in high school when I had to write essays in philosophy for example.
I like to write but I have to be content with prose or maxims.
The feeling of thinking is often enough on its own. All books will end in silence anyway.
Here is a cute seal.