Here`s some stuff...
Swimmers are known for their big lungs, so let`s do what they do: Crawl with an easy pace (i.e. not slow), breathe in the normal 3-strokes pattern (one breath every three strokes). Do this for 50m, then change to breathing every 5th stroke for 25m, then every 7th, then, if you`re good, go for 9 or 11 strokes, keep the last one up for 50m and go all the way down the pyramid again until you`re on 3 strokes for 50m again. Don`t pause until you`re fully up and down the pyramid once. Doing this a couple o`times a week should help.
If not, here`s another: You crawl 3*100m but you`re only supposed to breathe on the first 12.5m of a length. Breathing 12.5m on slow pace, then 12.5m 90% pace without breathing. Make some minutes of pause after every 100m to get that lactic acid out a bit. Hope this helps, it requires a bit of swimming endurance, but you can always shorten the exercises until you feel comfortable (or make them longer if they become too easy).
Have fun, greets, joe