Breathing is definitely the best first-aid approach.
As recommended by @godsol do check Wim Hof approach. It's great starting point.

You can continue with this podcast. More in depth from neurobiological side of things... And oh so interesting.

Personally I'd say the key is balanced eating, breathing and exercising daily. It takes time and focus and we all have to deal =/face the ups and downs... but it's well worth the effort.