I have asthma too.
It can also be exercise induced.
It can be allergic response.
Your lung capacity is reduced.
And your oxygen absorption is reduced.

You have to realise that your blood oxygen levels are effected.
I used to turn purple and blue because I could barely get oxygen into my bloodstream.

You have to start slow, and warm up well, gradually increasing in difficulty.
I noticed when I jump right in without warming up my asthma will kick in strong.

All dairy products will cause mucus and phlem.

I broke away from those ventilator prescription medications, those will kill you.
I now use herbs an hour or two before I train to avoid asthma attacks.

I also use Yoga/Chines Yoga (Chi Kung / Qui Gong).
I only use the blood oxygen saturating breathing exercises, and I do those while I'm driving to the gym.

You can also use certain breathing yoga drills to fight an attack.
I use the relaxing breathing drills during an attack.

You should work on building your lung capacity.
And work on calming your body during attacks, breathing slow and deep.

Oh, an don't forget you can be easily acquire lung infections/neumonia/etc.
I've had several bouts.
Constant/continual flaring/irritation of lungs can lead to that.

There's alot more to say but it is quite complex and difficult.
It took me my whole life to fight this.