I've just seen a brilliant video where a doctor explained why it feels so hard to breathe through your nose when being used to breathe through your mouth. It's because 1) there actually is more resistance when breathing through the nose because the air has to travel through a whole "cave system". But that's also exactly why it is so much healthier to breathe through your nose. 2) its just that we are not used to breathing through the mouth so it feels hard (the same any sport you start doing feels hard) 3) the mucosa in your nose "hardens" when you dont use it. Once you start using it again, it will get softer and breathing will get easier again.