I agree with all the pointers that have been posted.
When I was learning how to swim, I could not swim a length without feeling winded, and took long rests. I didn't know how to breathe.
I practiced and practiced. I would swim for a short distance while practicing my breathing. And one day it clicked. I was starting to swim for longer and longer periods, and over time, breathing was no longer an issue.
It came down to being comfortable being face down in the water, and being relaxed. Just before starting my workouts, I spend a few minutes with my breathing - exhale, blowing slowly into the water (mouth underwater), (inhale) taking short gasps of air (mouth, or nose, out of water), and repeating.
By regulating your breathing, you, in effect, can focus on body relaxation, and swim mechanics.
I swim based on the Total Immersion technique...and is what has helped me be able to swim for longer periods and distances. It is what got me through 4 Ironman-distance swims....2.4 miles.
Good luck with your training.
https://www.youtube.com/results?search_query=total+immersion+swimming