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.
“Peace is within oneself to be found in the same place as agitation and suffering. It is not found in a forest or on a hilltop, nor is it given by a teacher. Where you experience suffering, you can also find freedom from suffering. Trying to run away from suffering is actually to run toward it.” ― Ajahn Chah