I used to do box breathing. It really helps for anxiety attacks.
Last year I found the perfect anxiety relief for anyone who is autistic. It's called Qi Gong. It is a form of moving meditation that helps to lower anxiety. It is perfectly made for autistic people in the way that it lowers your stress levels with movements like shaking, body tapping, etc. . I could never do meditation before. While it helps I find it to be boring. Moving meditation on the other hand, gives your mind something to concentrate on while you do a simple, gentle workout.
I have been doing qi gong for a little over a year now and my breathing is better than it has been in more than 20 years. While I still have anxiety, I haven't had an attack in many months now. I have also lost a lot of weight this year while doing qi gong.
In the end, qi gong helps you to learn to breathe correctly, from your stomach rather than your chest. So it is also about breathing as well.
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No power in the 'verse can stop me. - River Tam (Firefly)