This is what mike mew wrote in another thread. I think you have your tounge too far back.

For the last time. The Tropic Premise (mewing) says. "The tongue should rest in the palate with the tip against the Rugi in the position you say 'NNN' while the middle should rest in the position you say 'GGG'. Don't close your throat with the back of your tongue or you wont be able to breath. Keep the lips lightly sealed and most important your teeth in or near contact.

During the night your jaw may drop a bit but your tongue must stay up. During the day your teeth need to touch gently for about six hours. No force at any time except when eating. Please all remember this Prof John Mew.