How do I fix inconsistency with my voice?

ThatK1D03

Recently or all the time, I have days or weeks where I can sing with no problem like no tension, no dizzyness, no drynees, no strain, etc. But then theres days where I can't even sing a simple song (well songs that were simple) For example, theres this one song I use to see if my voice is "perfect" the day I'm planning on singing. The song is For The Life Of Me by Daniel Kirkley and in the chorus it goes "Oh Lord" and the "Oh" is a 3 second E4 and the "Lord" increases to a 4 second F4, and theres days were I sing this 7 second phrase with ease and then theres days where I have trouble getting to the E4 or F4. I try to go back to my checkpoints to see whats wrong but it seems I can't find the problem, the days where I can't sing, includes me sometimes getting dizzy, sometimes my throat hurts from drynees or strain. Also theres a G4 or G#4 in the song, which on VERY good days I can hit. Can someone please help me on this, this is really stressful because it also affects my workouts, theres also some great workout days and some terrible workout days. -Frank
Edit: One day I was on xbox with my friends and I was singing and giving it my all, and I was singing Here With Me by MercyMe and I sustained the F#4 perfectly fine, and now I have trouble doing the same thing when singing that sing or in my vocal tract shaping workouts.

Wigs

Everything that you are saying points to still having moving targets and not getting a level of consistency with the fundamentals. I recommend posting your scales in the student area, for us to listen to. This is much better than a song because its easier to hear exactly what's going on and when singing songs, stylists choices can get in the way of technique.