First part of verses ok, but second part lifts while I fall!

This morning I was brushing my teeth, and sort of very quietly singing the beginning bit of Downtown to myself. I only did the first bit that ends in “downtown”, you know? Anyway, I didn’t do the rest cause I just KNEW it’d sound bad. I then was gathering a short list in my head of songs like this where I can sing certain parts ok, and it feels right and in control and not too difficult, but then a later part of the song, when it requires more oomph, power, RESONANCE really, I just lose it.

Like in Downtown for example…when she goes into that next bit with more words that are a bit quicker-sung than the first bit, I just lose it. Even in my head, I can feel the singing sensations move from the relaxed feeling of the back of my throat and mouth (which I feel when singing the first bit of the song), to pushing up behind my nose and the back/upper part of my throat, and it’s this dead, quiet, “blocked” sound.

I’ve mentioned before in some other thread that the Grace Slick song Let It Go is the same. The first bit I’m alright, but then at some point in the second part of the verse, I lose it and she soars and I fall flat.

Also White Rabbit, also by Grace. The first bit I’m fine. Then when she gets to the bit where she says “one man on the chess board” (or whatever), I lose it!

Can anyone recognise what this is? Like, what am I lacking right now that is causing this sort of problem? Am I supposed to be singing in head voice or something and I’m not doing it while these singers are, and that’s why they get the lift and projection but I don’t? Hmm.