that's not just you.
It's me and everyone else I know.
That's why these are called whiteboard firing squads.
The only way I know to improve on whiteboard interview performance is to do more whiteboard interviews unfortunately.
I know it does not make sense. I know it does not teach your or help you further any actual skill you will actually use day to day, but that's the problem we programmers have painted ourselves into.
The "remote" equivalent to whiteboard interviews are "live coding" sessions where you type code into a site like codebunk.io
Some random person who works at your potential employer will send you a link to a site you can type code into and ask you to solve a "5 minute problem". It's not fizzbuzz, it's not a for loop - it's going to be something slightly tricky like finding the sum of 3 numbers that add up to a specific number.
One thing I can recommend at a whiteboard firing squad is to see whether the firing squad has people in it that want you to win.
If you think out loud, show them you know what you're talking about, they might come out, feel your issue and work with you.
Isn't that the kind of people you want to work with regularly anyways?
I am happy to work out some trial whiteboard interviews with you if you feel that would help.