Kris, singing higher requires the use of your muscles in a different way.
Singing low: Vocal chords more relaxed, vibrate further apart, require more air.
Singing high: Vocal chords more taut, vibrate closer together, require less air, but under more pressure!BIG MISTAKE: To strain in the throat to make high notes.
RIGHT WAY: Create the pressure for the high notes by using the diaphram and the muscles in the back and legs. (I squeeze my buns together for the really high ones: perhaps I go too far...) Anyway, that's what they call breath support.BOTTOM LINE: You must get your power from the muscles below to sing the high notes, and be completely relaxed in your shoulders, neck and head.
I think most voice students will agree....my best to you!
Margarita