Your nasal passages are swollen shut. Try 4-6-8 counting.
Beathing in through your mouth still works but I purse my lips to restrict the air flow while I'm doing it.
Count slowly to 4 while breathing it in, hold it for 6 counts, then exhale for 8 counts.
It replicates your breathing when you sleep so it signals your brain you're sleeping and slows your heart rate as you calm down.
Try it and post about it.