M.C.
The answer might not be to supplement with formula- some babies are skinny some are chunky- and there is nothign wrong with you are your breastmilk or your baby. I have a little niece who is almost 3 and she barely fits into size 2 clothes- but nothing is wrong with her- that's just how she is. I also have friend who's daughter weighed about 18 pounds at 2 yrs and even though she was small there was nothing wrong with her. If she seems healthy and happy and is meeting all of the developmental milestone, there is most likey nothing "wrong" with her.
Another thing to remember is that those growth charts are based on the growth of formula feed infants- those babies can be bigger but it doesn't mean they are healthier. Formula fed babies are at higher risk of developing diabetes and being over weight later in life.
The thing with formula is that it is only 30% bio-available where as breastmilk is 100% bio-available- not a good choice to use to "fatten" up your baby considering your baby only gets 30% of it- the rest is junk fillers that no baby needs.
If you have your baby on solids by now I would suggest getting high protein healthy foods that also have healthy fats in them too to help with weight gain- it might be hard since 9 month olds are on a limited diet anyway-
You might also want to seek out a second opinion from a pediatrician who is more breastfeeding friendly and see what they have to say.
And another main thing is to try and end the self criticism that you are experiencing. Just because she was born by c-section and your dr says your breastmilk isn't "good enough" doesn't mean that you are letting your baby down or that your body is failing in some way. This kind of thinking can be self destructive and actually be a cause of the reduction in your milk supply.
My suggestion is for you to trust your body to do what it needs to nourish your baby and trust your baby to grow as she is supposed to. All kids grow differently and in their own way. If after a few months your daughter is still not gaining weight AND also showing developmental delays- then additional tests might be needed.
M.