For me it was a bit of a combination.

Main one was the breathing exercises in mediation. Which teach you how focusing on breathing can help steady yourself if tension, anxiety, nervousness, etc overtake you.

Secondarily was i was taking some Pilates classes. Seeing how breathing was important during movements helped reinforce the notion. Plus it helps you get more oxygen in and you tire less

Overall it was retraining my body to keep breathing :)

I'm onto the same track, there's something called myofunctional training/exercies which seem related but I've yet to try it. Would love to hear others opinions on it.

Would love to know as well. I often find that I've been holding my breath for quite a while whether I'm reading or working and suddenly will gasp for air.