There are many breathing mistakes people do while they stretch like holding their breath, breathing quickly or too deeply, inhale and exhale at the incorrect time that can really affect your exercise and even cause significant problems..

THE CORRECT WAY TO BREATH 

When holding a stretch, you have to take deep and regularly breaths.

When you breathe deeply, your body receives more oxygen and this increases oxygenated blood flow throughout the body and to your muscles.

Deep breathing also calms your nervous system and helps your body relax.

Anyway, try to breathe naturally while stretching and don't force deeper breaths.

You can use your breath to increase the intensity of your stretches.

On each exhale, move into your stretch, increasing the intensity of the stretch without forcing your muscles, and hold the stretch every time you inhale.

In fact when you inhale your muscles naturally contract while they relax anytime you exhale.

This makes it easier increasing your stretch, with less risk of injury.

While stretching, breath-in slowly through your nose, expanding the abdomen and exhale through your nose or mouth.

@findingthedivine, Instagram Yogi

You can also try to meditate during your stretching session!

Put a relaxing music on like this one, for instance, and try to relax.

Keep your eyes closed and try not to think about anything bringing your attention both to your breath and to the exercise.

To read more about meditation practice you can check the article "Meditation - How to get started".