Originally Posted by madvet
"breathing heavily the entire way"
Relax especially in your chest. Don't force the breath in or out excessively. Still breathing hard? Relax even more. Etc. Strive to breathe as if you were going on a moderate walk. Sure, your time might temporarily suffer. But you can't move forward without fixing this.
I was going to mention this before. The breathe LESS theory. One thing that is helping my breaststroke, for example, is not inhaling more than I really need to. Inhaling too much causes more tension which tires me out. Getting the fuel/air mixture right helps. I don't move fast or go far, so I know I don't need to blow up like a puffer fish.
Was thoroughly exhaling covered?