Hi @gabrielm,
Thank you so much for your post. Myself and my younger brother have both dealt with this same issue for years and both of us have come to a shaky conclusion or hypothesis. I'll give you my story and his as briefly as I can.

Mine started when I was in high school and working as a lifeguard. I got sick and had a cough for at least a year. During that entire time, I had terrible shortness of breath. It was so bad that I was prescribed a medicated inhaler from the doctor to help with the SOB. I don't think it helped very much and seemed to stick around forever (he seemed to think I had "temporary asthma"). I learned several years later that another lifeguard had SOB for about a year during the same time that I did, and she believed it was because there was an Ozone leak right near the lifeguard stand where we sat for several hours everyday. Who knows if that had something to do with it or not.
Fast forward about 10 years and I got a new and very stressful job. As I was coming to a very crazy time in my job, I began getting a shortness of breath again. It was so bad that no matter what I was or wasn't thinking about, no matter what sort of breathing exercises I did, or cardio exercise I did, I could never get that "deep breath satisfaction" of feeling satiated. This was all compounded by coinciding faithfulness issues on my husband's part and I started having panic attacks. This went on for several months. I was convinced it was anxiety but over the past year I have become less convinced of that because I no longer have a stressful job and my marriage is great. I am not at all an anxious person and yet I still (three years later) occasionally, for a day or so, and sometimes for a week or two, have SOB. I'm currently suffering with it and it all started right after finishing a morning run this past Monday.

My brother's issue started because he had undiagnosed mono and was powerlifting at the same time. He believes he injured his throat while lifting and that compounded on top being ill and over-prescribed penicillin for a "sinus infection" (which he didn't have--it was the mono). He went through a similar flurry of doctor's visits like you, and everyone had a different theory. Most told him the SOB was anxiety, but he wouldn't believe that. Finally he found a doctor who mentioned he could have vocal chord dysfunction. The little bit he has told me (I haven't read about this myself) is that a lot of athletes get this from being injured somehow in the neck/throat area (like football players). The vocal chords and throat are damaged, and from that point on, the chords randomly "act up" and the individual feels like they can't breath and experience SOB. There apparently is not really a cure for this issue.

All that is to say, I wonder if your acid-reflux (or some other incident in your past) has damaged your throat and now you lungs/throat area is "oversensitive" and easily irritated by external or internal stimuli. I wonder that for myself--if past damage to my lungs has basically made them incredibly sensitive and they randomly "overreact" to stimuli by "thinking" that they aren't getting enough oxygen. Basically, they were damaged in such a way that they (or a part of your brain) can't tell if they are supplying your body with enough oxygen. Like I said at the beginning of the post, this is a shaky hypothesis based on my own experiences and things I have read online and not based on my own medical knowledge past college level biology and anatomy. Maybe the SOB is some sort of neurological issue tied to our physical lungs/throat/brain area that is out of whack and no longer in alignment.

I hope mine and my brother's stories are somewhat helpful in your search for answers.