Sorry it is so hard for you!!!!!
All I can do is remind myself that even though it feels like I can't breathe, my body will manage to breathe. It is a feeling which is very scary but you will keep breathing.
I concentrate on breathing out as slowly as I can ( breathing in happens by itself but sometimes we forget to breathe out and that makes the chest feel tighter). The out breath activates the calming part of our nervous system.
Then the only thing to do is to step outside the door, but maybe only a short way and take your time. Can you sit somewhere and do a distracting puzzle or some other activity until your body calms down. It will do...eventually.

I really feel for you.
Wishing strength for you. Remember no one is brave unless they feel scared. You're braver than you know!