A good breathing exercise would be to practice expelling the least air possible while holding a note, or singling a long phrase. You'll notice that when you reduce the exhalation to a low enough level, the note will collapse. Just before collapsing, the note will go unstable (vocal fry). The goal is to keep the outward airflow just above the fry level, so you get a quality note and conserve air to sing efficiently.
The other thing to practice is the inhalation breaks between phrases....see how long it takes to inhale to full lung capacity, starting from 3/4 full lung, or a 1/2-full lung. To keep your brain at tip top oxygenation (which will keep you calm!), the idea is to fully expand lungs on each inhale, even if you only exhale 1/2 the air on the next phrase. Why? The fuller the lungs, the higher the oxygen partial pressure in the air sacs, the better O2 uptake to the bloodstream. If you go the other route, and tend to only fill your lungs partially during inhales, the average pressure in the lungs will be lower, and you can suffer low O2 saturation, which causes a panic reaction, a feeling like you can't breathe and might pass out. Obviously, that breathing pattern is self-defeating. Fill lungs to capacity on each inhale (that is, after your soft-tissue injury heals and you can do so without any pain!!).