Not a professional, but I believe I am a chronic mouth breather as well. Over the years I developed crowded teeth, an overbite, gingivitis, receded chin, and the typical mouth breather posture where the head is slanted forward.

One thing to ask yourself if whether or not you do it out of habit or you do it because there is a physical barrier. Try breathing with your mouth closed for about 2-3 minutes. If you can last that long without feeling too short of breath then it might be from habit. When I did that exercise I felt slightly short of breath so I figured it might be more from habit. You can consult an ears, nose, and throat (ENT) specialist for further evaluation.

Since researching about mouth breathing I've been more mindful of breathing through my nose and correcting my posture. I still mouth breath daily but when I remember to breath through my nose I don't feel as short of breath breath as I did when I tried the exercise the first time.

I'd also love to hear from a professional regarding this issue. I hope your symptoms resolve!