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.
"Right effort is effort with wisdom. Because where there is wisdom, there is interest. The desire to know something is wisdom at work. Being mindful is not difficult. But it’s difficult to be continuously aware. For that you need right effort. But it does not require a great deal of energy. It’s relaxed perseverance in reminding yourself to be aware. When you are aware, wisdom unfolds naturally, and there is still more interest." - Sayadaw U Tejaniya