I found there was so much pressure to do everything on demand when DD was born (now 7yo). I had an awful book called "The womanly art of breastfeeding" which made me feel dreadful, it was all about being a martyr mummy and never getting a break from your baby, doing everything constantly on demand. None of this actually helped with breastfeeding, I still had low supply.
. I developed severe PND too.
On yet another miserable trip out where I fantasised about going back to work that second, I happened to see a book in a bookshop called "The baby book:how to enjoy year one" and bought it as I was absolutely loathing year one at that point!
It was similar to GF about routines and structure. It made things seem doable. We did the suggested routines and it made such a difference - baby slept through by 8 weeks, I was more rested and happier (this actually increased my milk supply!). Each day had a predictable rhythm to it.
I got a lot of stick at the time from some anti-routine people when I said I'd used it. But I now have a happy confident child who still sleeps and eats well, whereas I know a lot of kids the same age who were on the demand-led approach who eat really badly, still rake in the night, don't go to bed until really late.
TBH I wish I'd used GF from birth or the book I did find. If you're a person who likes structure and routine, the demand-led stuff is way too stressful and uncertain.