I think you are likely a fantastic scrabble player!
We all want to beat the champs.

I used to play in cribbage tournaments, some 30 years ago. I would win then.
I'd just started playing again this winter. First, I had to learn all over again, as I could not recall how to play. My goal was to be able to partner with my mother in cribbage tournaments, as she is very good at this game. I used to be able to play all day long and would stay "sharp." I now start losing my focus and want/need a nap before I've finished a match. I lose focus and start making quick dumb moves. I do that as long as I have lost focus. I do okay as long as I can keep my mind on the game. I can do it, but it's not as easily done as it used to be. May be age, drugs, depression -- any or all of the above.
Scrabble -- at the level you are playing -- is very difficult. Don't be too hard on yourself! You are obviously a formidable opponent!

WC