Let me throw my 2 cents in. Alot of time it just takes practice. If you do not have access to a pool or body of water, just practice in your tube, a little cramped but can be done. I would not try to get through this by holding you nose or using nose plugs. It may seem like a sensless skill but I can tell you I have had my mask pulled off by a strong current, it has been kicked off by other divers fins, the mask strap has broken. All this over alot of years of diving, alot of it tech diving, but some of it just easy dives. This is one skill I always stress that must be learned. With practice you can overcome this, I have only known one person who could not/would not. Most people breath through their nose so it is hard habit to break. Keep trying.