Thank you for the comments. I definitely think there is tension buildup as I get higher in pitch. From what I can tell, and from what I know about the voice, I think I am pushing my jaw forward as I get higher in pitch which is causing tension to build up, and is forcing me to use even more air against my cords to sustain the notes (which I think is causing the fatigue and probably cord swelling as I get through song, especially those that are at the upper end of my middle register).
I am working on the following (and you can correct me if you think anything is incorrect):
-Keeping the jaw more relaxed so that I can place it down and back as opposed to jolted forward
-Keeping the tongue relaxed touching the bottom teeth so that I don't pull it back on higher notes (I find that I do this, as well as when I breathe).
-Be more conscious of my breath support (I sing right now however my body naturally allows me to, which doesn't seem to be working, and I don't think I am supporting as much as I could).
-Keeping my placement more forward - I think my last note, which you disliked, was almost swallowed (almost as though I am mimicking what a classical sound should sound like rather than just letting it occur naturally).
What I am having trouble with:
-Keeping my upper notes (not head voice) powerful but with no tension. I know this is done more healthily with a mix, but even though I can sometimes get a mix when doing voice exercises, I can't seem to access it when I transition to song. I have done slow siren exercises from the very top of my head voice all the way down to my lowest chest note without breaks, yet when I am singing a song, I can't transition into a mix - I either chest voice a note or I head voice it.