There are so many things that could contribute to her being sick, so don't beat yourself up over it. You've done so many great things to try to promote health, but breastfeeding, good sleep, probiotics - none of them are 'magic bullets' that will keep people from getting sick. Yes, they'll certainly help but kids get sick. It's a fact of life! Please don't think that because your kid is getting sick you've failed - it really has nothing to do with that (I say this not because I think you have failed; I say this because a few of my friends felt the same way, like a 'where did I go wrong?').
The only thing I could think of would be limited contact with other children. If she hasn't spent a lot of time with other children, she may not have been exposed to different illnesses and her body might just be doing a little bit of catching up. Although I have no scientific basis for this casual observation, among my group of friends more of the kids of moms who worked noticed their kids getting sicker younger and more of the kids of sahms noticed their kids getting sicker later. My kid started daycare at 9 months and immediately got sick. His first year in daycare he got sick a lot. Now he is in his second year and he's been sick once, despite the fact that this has been a pretty treacherous winter illness season. Again, I'm not stating this as fact and certainly it could go the other way.
I know it must be difficult to see your child so sick, but just keep on doing what you're doing and keep on giving her that TLC! Fortunately spring is just around the corner (let's hope so; this cold and snow in April sucks!).