Whenever I've taught a newbie to knit...tight cast-ons and tight knitting kinda comes with the territory. I think it's nerves at bit.
But here's what I've found to be a great remedy:
I teach them to cast on LOOSELY. If they can't quite cast on evenly and loosely,
we go to PLAN B: using a larger needle size for the cast on.
Tight cast ons result in tight knitting for sure.
If the cast on row is loose (not sloppy, just loose enough so that the needle can slide back and forth inside the cast on stitches), the knitting will be LESS TIGHT than otherwise.
If you are gonna use, let's say, a US8 for your next swatch...then cast on with a US10 and keep the tension just as tight as you want.
But when you begin row 1 of the swatch, use your US8.
If you don't like that, then use a US9 to cast on, and the US8 to knit.
Swatches are so helpful!