USDA, don't feel alone. I've had a bout of dyspnea ( difficult to catch your breath ) since the fall of 2017 after I caught some mild but lingering 4 wk long bug. Got really bad that winter. Feels like you can't get that deep breath, then when you finally do, the cycle starts all over again. Blew my $3K deductible on several doctors where nothing was found, and its not silly "anxiety" as the first quack suggested, where his mind was made up before he examined me. I'm seldom anxious about anything and when I am, I can actually breather better. They never figured out what it was, and I still have it, tho not as severe. You learn to ignore it. In my case, I'm getting enough O2 ... just don't think every part of my lungs are, and those are the areas producing the feeling. I also have a mild cough associated with it, so phlegm is a factor somehow. I don't smoke, BTW. Also worth mentioning is that I've had this on and off my entire adult life, about every three or four years it would flare up for no reason and last for a couple weeks each time. This current bout, however has been around in some fashion for going on 3 years now. Some episodes in the past when younger I was pretty sure were related to hunger, where eating something cleared it up. Blood sugar problem? My blood sugar is normal, BTW.
In researching this quite a bit, the one thing I've found out is there's no single root cause for this. Medical (pseudo)science lumps all of them under one term: dyspnea. They don't understand it nor do they care enough to find a cure for it. So long as O2 sat is mid 90s or so, they don't see you in medical need. It can be very uncomfortable and even frightening at times. Find out what works for you and use that. For me, it's ignoring it.