ur doing a mistake by directing the air to the windscreen. u have to cool down the cabin but dont direct the air to the windscreen. if u do, the particular spots on the screen get too cold and get frosty. just turn the a/c on and direct the air towards urself. give it some time so that the cabin is cooler, the fog will disappear slowly.
one of my cousin used to do this mistake in his civic. after a few days i asked him to let me control the frost issue n then it never happened again n onces u direct the a/c towards windscreen and frost appears, theres no going back, ur doomed. the difference in inside n outside temperature wud be too much (due to direct cool air supply to screen) n u wud keep in reversing the direction but it wud just reverse the situation, i.e. fog inside or fog outside.