I agree with the other comment. You can't spoil a baby. I'm shocked that the doctor and health visitor told you that it's behavioral. Babies cry to express their needs, they have no other way of communicating with us. Sure, some babies are most fussy and difficult but that's not because their trying to be difficult. They don't have the knowledge to do that yet. If you're feeding her all day, it's possible her stomach is starting to hurt. Breastfed babies can't be overfed but formula babies can be. If she's not napping then she probably gets over tired and then that causes her to be fussy. I'd keep trying different things such as: trying different pacifiers(dummys), seeing a lactation consultant, trying drops for collic, bouncing on an excersice ball with her, rocking in a rocking chair, using a baby swing, using a baby carrier, swaddling, singing to her, etc. Babies also feel our emotions so if you're getting frustrated she'll feel that and it'll be harder for her to calm down. I wish you luck!
You are only thing that baby has. The baby is not and can not be spoilt… love, comfort and trust is what your baby might be seeking from you & if your baby isn’t doing it when people are around then she’s getting stimulated and when they aren’t there your baby is bored. Download wonderweeks it helps you understand your babies needs.
A 12 week old can't be spoilt and can't manipulate you.. some babies just have higher emotional needs and want to be cuddled more and be closer to you.. it's a natural instinct - this world is new for them and you are their only certainty.. things will get better with time :) x