dear, dear A.;
first of all let me congratulate you on taking the initiative to reach out about this challenge in your mothering life. it takes a LOT of guts to speak openly about what troubles us most.
secondly i have to really, REALLY commend you, like i would like to give you a trophy, for staying committed to nursing and breastmilk only; without a doubt it's by far the BEST THING you can do for your child, and you've been doing it! especially in a very medicalized atmosphere, mothers are really subverted and diverted from the profound benefits of breastfeeding, and it's just fantastic that you have stuck with it.
i'm sorry that it sounds like you haven't been aware of La Leche League Intl; if you have, then i'm sure you've exhausted thier information and ideas. but if you haven't, it's not too late to look for this organization on the web easily by googling it, and going to your local chapter meeting or calling the chapter leader for your area. these are the best, most up to date informed people on infant nutrition and nursing. even now well into your child's second year they can still help.
i do not have experience with what you are dealing with, and you are, very obviously, smart, sophisticated and resourceful, but i'll throw out a couple of ideas to you for the hell of it.
first of all; isn't it possible that you just happen to have a very neatly packaged little person here? some people are tiny! i remember being the big child in the class, always standing in the back row for pictures, having to buy clothes in the preteen and husky department by 11 years old, and being terribly envious of these birdlike little girls that were so delicate and springy! i mean TINY little things that were very clearly bright, healthy, active, normal kids.
maybe one thing you could do is talk to moms you see at the playground with slightly OLDER children; in other words, consider this from the other end; not where your girl should be by now, but where she could be in 4 or 6 years from now. when you're at the playground, look for short, skinny kids and find thier moms, and be frank, be friendly, and go ahead and say,
"Listen, could I ask you a mom question? is your child petite? because my kid is very small and i'm looking for another perspective, beyond the growth chart expectations. what was your experience like?"
or something like that. because you know, we all love to talk about this stuff, and chances are the mom will be delighted to recount for you what her fears were like and how she navigated and survived the situation.
then too, what about putting her in a mommy-n-me class with you? not with any other caregiver, but just with you, so you can really see how she's doing compared to other kids, with other kids, and give yourself a chance to get to know some other moms that you could talk to.
the next thing i would say is, if your child is developing normally in other ways, maybe it's time to take a MEDICAL BREAK. stop with the doctors, just for a while. give yourself a boundary, say, for 3 months, don't go to the doctor to pursue this particular issue at all; only go if your child has something obvious and urgent like a bad cough or fever. just break this cycle of feeling beholden to the doctor all the time. and see what happens, you know, give yourself a chance to breathe and let the anxiety out of this.
laslty, if you haven't already i would say check out Askmoxie.com and Kellymom.com.