This came up recently on Reddit; due mostly to anxiety, a poster could not write without constant pacing. (PSA: Constant pacing is pretty much incompatible with typing a story!)
I don’t get anxiety but I am a professional author, so I offered some purely technical workarounds in descending order of probable usefulness…
Option 1: Don’t type
There is a very successful writer – Kevin J Anderson – who dictates his stories while walking the hills then gets them typed up. There is also Dragon Dictate voice recognition software which a pro friend of mine swears by.
So, rather than fight to stay seated and keep your hands on the keyboard, you could get a recording device and pace to your heart’s content! You’d need to feed the results through Dragon Dictate or type them up later.
However you may find typing up from a recording is easier than typing and creating.
Option 2: Don’t sit
Use a “standing desk”.
A variety of adjustable desks are available that let you stand while you type. You can also rig something up yourself. This would let you shift your weight around and so on, or at least pace away from the keyboard then back without then having to get in and out of a chair.
If you have the money, you can even get a “Walking Desk” which is basically a desk with treadmill.
Option 3: Learn to touch type
I’m betting that you don’t quite know how to touch type. If so, then I wonder if learning to touch type might adjust your cognitive load down just enough to make it easier to spend time writing.
Option 4: Take a break and feed your writing
If the current intensity of your symptoms is temporary, rather than fight it, perhaps you could instead consciously feed your writing for later. I mean read/listen to inspiring books, research settings, visit settings, get or observe new experiences… all to feed your creativity.