A breathing technique I've tried, that works well against physical pain, and is excellent for calming down your nervous system. I call it the LSD breath, (Long, Slow, Diaphragmic):
Practice looong exhales, followed by breath holds, not too long so that you gasp for air on the next inhale, but long enough to create a mild air hunger. You'll have to play with the duration and find your sweet spot (it will vary). Shorten your inhale, but remain comfortable. You want a mild sense of air hunger, over time, your body will adjust.

Use your mind during the exhales and breath holds to invoke a sense of peace and relaxation.

After a while you'll start feeling tingling in your body, carry on and you'll later feel a deep and profound relaxation.

Best done in bed. Give it a try and let me know if it works for you as well.

(If you need a shirt break at some point, have a ten to fifteen seconds break, and get back to it)