Now I am no expert, but I have had some vocal training both in HS and college. What you have reached is called your "passagio" or lift voice. It is the point where you have to make an adjustment in your voice to reach a higher note. Many people sometimes sound like two different people, others work very very hard to eliminate or greatly diminish how noticeable it is. I noticed that you said you can sing up to G above middle C, What is your lowest note that is comfortable for you to reach? I could give you more specific information based on your current range.
Now as for singing from your chest, you really should learn to use the "mask" of you face to resonate your sound. What I mean is that you focus your tone upward (not nasal). I use pictures to teach this to our praise and Worship team so it is hard to explain without pics. Try singing the word "hung" and hold the "ng" for four counts and gradually get louder. Make a note of where you felt the vibrations in your face. Next, sing the word "hung" and immediately drop your jaw and sing ahhhh. So it would sound like "Hung ahhhhh" Each time you do this make sure that on AHHH you are gradually increasing your volume. I hope this makes sense to you. If not just let me know.
YOu may want to invest in some singing lessons with a proffessional teacher to help you focus your tone. That way you don't damage your very fragile vocal chords. And you will have a great understanding of how the human voice works.
Nichole
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