I can't make medical recommendations here, but I can say that people I have worked with who discuss similar issues often do well by starting with cyclic sighing and other body relaxing techniques.

In this article I talk about it for sleep, but it has been employed more extensively for daytime anxiety, and I have YouTube videos discussing breathing in the context of ADHD.

https://medium.com/wise-well/an-amazing-sleep-aid-cyclic-sighing-e9d5717e2df1