Thank you for sharing your query here.
In the first instance, I would like to suggest you to take deep breath 5-7 times before attempting the posture of half moon and while attempting, follow the below principle:
Exhale while forwarding and inhale while backwarding and once you hold your posture of half moon, do not hold your breath.
Slowly increase your stamina and do not attempt the posture beyond your capacity.
Please follow the above and let me know if you still face such discomfort, so that I can suggest other points.