I'm sorry you've had such a struggle, OP.
Are there any local Baby Cafes or other drop-ins where you can talk things through?
Breastfed babies are not 'healthier' than formula-fed babies. They are the biological norm (as in, that is the way human babies are evolved to feed). Therefore, it would be more accurate (but contentious) to say that formula-fed babies may be at risk of being unhealthier than bf babies. They are more likely to get infections, stomach upsets, allergies and so on. They won't have the antibodies that cannot be replicated by formula, but which are in breast milk. Breast is not 'best', it's 'normal' for humans.
Your DD - or any one baby - might have a genetic predisposition towards all sorts of things - eczema, asthma, being affected quite strongly by viruses, etc - breastmilk isn't going to reverse that predisposition but it will protect against getting other illnesses or infections.
And all babies are individuals, so while I really appreciate your struggle, it's a bit meaningless for you to say she is doing 'worse' than your formula fed son. He is different from her, they are two separate children with unique genetic make-up.
Sorry if I've offended anyone here. I'm a bf peer supporter working in an independent infant feeding drop-in (we welcome ALL parents and babies, incidentally, we're not there just for breast-feeding). I try to tell it like it is because the whole 'breast is best' line does nobody any favours. Breastmilk is the biological norm.