The body won’t voluntarily stop breathing, it knows what it’s doing and will be getting enough air.
The mind thinks it has to be in control, and panics when it thinks it’s not getting enough.
I think you’ve got an interesting situation here to learn from, you may have to watch this happen again and again until you can learn to let go and trust in the process. Just remind the mind it’s here to watch and learn, not control a bodily process that is automatic anyway.
"Proper effort is not the effort to make something particular happen. It is the effort to be aware and awake each moment." - Ajahn Chah
"When we see beyond self, we no longer cling to happiness. When we stop clinging, we can begin to be happy." - Ajahn Chah
"Know and watch your heart. It’s pure but emotions come to colour it." — Ajahn Chah