I don't know

I wish I could help, but don't know if I can.

I don't suffer from serious anxiety, though I do sometimes stress about reaching my standards and goals when a deadline looms.

What I'll share comes from Buddhist meditation - an awareness practise which requires no belief.
Try sitting in a quiet place with your back straight and supporting its own weight. Take a few deep breaths and observe the feel of the air as your breath comes in and goes out. Eyes closed.
Begin to scan the body systematically, noting whatever sensations you feel. Start with the scalp, forehead, eyes, eyelids, cheeks, mouth, jaw, joint between cheekbone and jaw, tongue, throat and neck. Take a moment to consciously allow each of these to relax. Scan them again, just observing.
Continue like this down your back, and the front of your body, down each arm, elbows, wrists, the hands, fingers, down each leg, thighs, knees, calves, shins, ankles, feet, heels, arches, toes, soles. Note where there are tensions in the muscles and let them relax. Scan again to feel and observe.
Now see if you can go inside the body. Do you feel the chest and stomach expanding and contracting as you breathe?
Is it slight and shallow, fast or slow, deep and slow, or is it variable. Does it change as you watch?
If thoughts arise, note that they are there, but return the attention to scanning the body and observing it.
Try to maintain this for twenty minutes if possible.
At the end, make a note of what you have observed. Put a time and date on it.
If you like the process, try it every day.

Another thing to try. Breathe into a paper bag for a minute or so. The carbon dioxide will have a calming effect.

There are ways of exploring the causes of anxiety. These can help to understand it, which can help to make it far less uncomfortable - but it's not a cure.

There are CBT (cognitive behavioural training) techniques for reducing and eventually eliminating anxiety in the long term. These are well proven and reliable.
People can work with a specialist psychologist, therapist or counsellor with training in the technique.
But it is also possible to just learn the theory and then develop a specific way of applying it your needs. You can do this for yourself for free.

Message me if you'd like to know more.