What are your breasts like after a feed? I just wonder as I had a C-section (+ 2 days in ICU in which I fed him only a few times) and was breastfeeding and DS was slow gaining weight too. I had oversupply from all the pumping I was told to do after the C-section because apparently milk doesn't come in without the labour but everything I did to increase my supply just backfired because he was struggling so much with the amount of milk squirting down his throat and not getting much fat and not gaining.
You're absolutely not doomed because you didn't feed her right away, I breastfed for 2.5 years and DS had formula for his first feed and a few supplemental feeds until we worked out I had too much milk, not insufficient.
Are you able to pump after a feed? Could you supplement with pumped breastmilk? Has she been checked for tongue tie, etc.? Has she had a weighed feed so you know how much she's getting and that she's able to transfer milk efficiently? Undersupply is not particularly common, so worth looking at other reasons she might not be gaining too. Are you feeding frequently? That will increase your supply.
That said, were you on a drip for a long time before delivery? Sometimes that can inflate birthweight and make it take longer for LO to get back to birthweight.
Sorry, that's a lot of questions, but hopefully something will click and you'll be able to work something out for you and your baby.
Here's some info that might be useful:
I'm worried I may have low milk supplyBreast compressionOversupply and Overactive (Forceful) Let-Down