To the OP, Is your current teacher helping you with this? Not sure how for you are moving your jaw forward. I would have to see and hear you play. If your current teacher isn't helping you, try another teacher. Get a second opinion. Not all teachers are equal. Some of them didn't graduate at the top of their class. If you feel your throat closing ( it doesn't) you could be arching the back of your tongue way too much. You may be doing this without realizing it. Do you get a nasal sound?