Swimming 40 s in 50m is a 1:20/100m pace, thatís excellent. Given your description on breathing difficutly is certainly the exchange is not quite right yet. The source of the lack of exchange is usually tight chest, shoulders and neck - even if youíre in posture. One thing I have discovered that helps swimmers reduce this tension is to breathe on oneís, swimmer dosesnít have time to get out of posture and must be relaxed in chest, shoulder and neck to breathe on oneís
What is breathing on oneís? You breathe toward shoulder on one stroke then breathe toward opposite shoulder on next stroke. Itís also a good strategy to get more air when you need in open water, and in the pool before the turn and after the turn. Sun Yang and Katie Ledecky use this strategy going into and bouncing off the wall too; and in the middle of the length when they need air.
Start off the wall breathing on twoís (breathing on second stroke in cycle), then occasionally try breathing on oneís just for two strokes or single stroke cycle. Donít breathe on oneís (everything stroke) for the length or more than two strokes.
Give it a try. If you get air on both left and right shoulder breathing on oneís, you will become aware of the air exchange issues that are leaving you breathless, and will also find you donít have time to be breathless :-)