Doctors sometimes suggest those suffering from asthma take up a wind instrument.
It gets you breathing... while not focused solely on the symptoms.
Many singers have to work around asthma (Suzanne Vega comes to mind).
I deal with mild asthma (though I don't use inhalers/etc).
I have to remember to breathe where I can (deep breaths if possible).
At first, there were nights it was tough getting thru a four hour show.
Now... it's not a big deal.
Had to drop the idea of playing Animals by Nickelback. That song is relentless when trying to breathe!
(Insert your favorite Nickelback joke here.)
Being relaxed promotes easy breathing.
Arrive at the gig with plenty of time for setup, have backups, and keep the vibe positive.