Wim Hoff Method- There are a lot of breathing techniques out there but this is the most simple. Meditate afterwards for extra effect
A course in miracles- This is a one year course designed to get you out of the unconscious overthinking patterns that plagues this world.
Autogenics This is a form of mental training. It was used by Ueli Steck one of the best alpine climbers of all time.
Holotropic breathing- You can pay to go on a course and do this with instructors or you can do it at home. But if you have any underlying emotional issues this can help resolve them.
Shamanic Breathing — So this is a pretty wild experience if you persevere. Effects are similar to a psychadelic experience and it can also solve deeply routed emotional issues.