First, try and identify why writing gives you anxiety. I think identifying the root cause of the stress and clarifying what is triggering you to feel this way is important.
Next, just take it slow. 15 minutes a day if you can. Little bursts are key.
Of course, I’m not a doctor, but I get some anxiety about writing and I find that’s what helps. It’s when I blow it up too much, or lose sight of the enjoyment of the process because I’m too focused on the end results when I fin my mind start to waver and slip.
Hope that helps. let me know if I can support you in any other way.