Hi.
I've been practicing mindful breathing for about 5 years now.
Forgive me as I don't remember a lot of information, like the whys (my memory is shocking) but I will try and explain it as best I can. I have ADHD and I'm in perimenopause so sometimes I struggle to get my words out in the right way so I hope I can explain my experience of it to you for you to understand 🫣
I breathe in and out of my nose mostly as opposed to in through the nose and out of the mouth. The reason for it (and this is where I get stuck, sorry) is because it stimulates (?) the pineal gland. It does something to the pineal gland and that's the thing to reduce the anxiety (see your preferred search engine).
I take a deep but fairly short breath in and I exhale slowly, controlled for longer (than I inhaled) and when I naturally feel to inhale again I do so without force.
There's no holding the breath or forced flow, it's all natural flow. If you need numbers then for example, you can breathe in for 4-6 and exhale for 8-12 (or as long as you can). You'll be able to breath out for ages eventually. It's pretty wonderful.
I listen to Buddhist teachings as I fall asleep.
An out of the mouth suggestion (can also be done through the nose only):
I also imagine that I'm breathing in a white smoke of light and love.
I try to visualise it entering my nostrils and travelling down the wind pipe into my body, like a magnet I send my feelings/emotions/anxiety to the white smoke and as I exhale I visualise it travelling back up towards my nose and try and exhale it all in one go.
I become aware that as my tummy rises with the inhalation, on the exhale and my tummy is going down it's like it helps send the anxiety up to my nose. Like a boost type thing.
If I can't release it all in one go then I repeat the same process, I'm still inhaling the white smoke of light and love only it's bigger than the remaining anxt.
I still pull up more anxiety/emotion when I exhale but the more I practice this, the more I exhale the shit stuff and fill myself with the white light and love.
When faced with an immediate negative reaction to something (other driver unfortunately 🙄), I regularly close my eyes and take a deep breath in and I exhale slowly out of my mouth (if it's safe to do so of course lol).
I hope something has been helpful for you.