So, first let me just throw this in. Why are you going to the ER for this? That's an expensive and unnecessary route to take. Get a primary care doctor that you can just see in the office for normal care rather than taking the ER docs away from emergencies and costing extra money because ER's are more expensive than a doctor's visit. Plus, ER's are set up to handle acute emergencies and not an ongoing issue like you are having. Once they see you and clear you that it isn't an emergency, they can't do much for you. So, just trying to direct you to the right type of medical care.
When we have a health concern, it is natural for it to sometimes get nervous about it. Some people have anxiety that manifests around their health and that takes it to a higher level. That would be when we have a panic attack or similar reactions to fear about our health. This is something to talk to a primary care doctor about(not ER) to start the ball rolling for next steps to address it. They are often the gatekeepers to any specialist like a psychiatrist that you see and also may have a list of counselors in their area that other patients of theirs have liked. You can try deep breathing exercises and meditation to help. You can try regular exercise to help. (that also may help your lungs clear of mucus).
I have a good friend with asthma and to this day, she does the occasional breathing treatment. She's not going to die of her asthma, she just makes herself more comfortable. If you need an inhaler, get an inhaler. But undue panic doesn't help anything. So, I'd look at this in terms of two issues to deal with. The phlegm in your cough/asthma if you have it (and some get a temporary asthma after a bout of illness which is different that regular asthma and I'm not clear which you had) and the second area to address that is separate is anxiety. good luck
Agree with the above, and if they say you don't have asthma, you probably don't have asthma, you probably get allergies and are probably lacking endurance because you don't exercise. I'd personally avoid pharmaceutical if you can for the allergies, they are either sedating or energizing, and see if you can't find some natural antihistamines that do the trick -- the natural world is full of antihistamines, and if they're strong enough, most of them don't either over-stimulate or sedate. But trying to start an exercise program, gradually working your way up, providing when you do get that regular doc who can evaluate you more thoroughly than they do at the ER nothing is found to prevent that, you might find you just need some more pep and exercise can provide that.