Usually when I hit songs that have long notes and are somewhat in my mid/high position in range my voice kind of tires out. I can sing the whole song again, but at these long note parts I just can't last very long.
My voice still kind of "tires out" till the end. I feel as if my muscles can't sustain that pitch anymore, and it just drops. I am using head voice when singing high notes. Now I can sing these songs but after or almost till the end I feel so strained. Sometimes I can't finish them.
Some song examples:
Tesseract - Perfection (ending)
And when I sing songs that even have a bit higher range (for me) but go up and down a lot it's a lot easier to sing for me like:
Tool - Parabola (high parts)