Swimming fatigue
A good way to hold off swimming fatigue is to change your stroke throughout your session. While there is some overlap, you use a number of different muscles with each stroke, particularly in the kick.
Backstroke is particularly good for catching your breath and steadying your breathing because your face and to some degree your chest are out of the water.
Don’t change your strokes too much because that can be counterproductive. Just rotate once you feel your technique dropping for one stroke.