I agree with retired guy, I always thought I couldn't breath through my nose, but after approximately a week I'd learnt to.
The way I learned was quite simple, using nasal pillows, just before going to sleep, I would consciously open my mouth and airway so I new what it felt like to have air coming out my mouth. Then I closed down the airway by closing my mouth and pushing my tongue into the roof of my mouth and told myself this felt good and to stay like this, a kind of positive mental self reinforcement.
Kept on doing this routine until it became second nature, it works so well that if I inadvertently open my mouth whilst sleeping, I notice the difference without fully waking and I automatically go back to sealing my oral airway.
The benefits of using a pillow mask against a FFM make it worth a little perseverance.
You can do it !!!!