For my DC's sickness, I blame myself as he was born via C-section and I wasn't able to breastfeed. He missed out on the antibodies which he good had got from breast milk. I don't even know at this point what to do, just feeling overwhelmed with all this constant illness he has been through since last year.
Please don't blame this reason! DD was breastfed for 2 years and I was absolutely broken by the amount of bugs she caught during nursery. The endless illnesses (and related unpredictability such as failing at work, missing deadlines, cancelling plans) took me to a place darker than PND to be honest.
I think it's just an incredibly shitty, difficult phase of raising kids that absolutely nobody really warns you about. The difference is that all parents have different benchmarks on how sick their kids needs to be to keep them home, or leave them in the care of someone else. Some parents aren't bothered at all because they just dump their sick kids into nursery and go about their merry way. If DD is ill and wants me then I can't bear to leave her or force her to go to nursery. So I still end up not being able to work even though we have a FT nursery spot and babysitters/grandparents as backup.
People will try to tell you it gets better, or you can somehow work around the illnesses but that's all bullshit. You need to accept they will get ill all the time, due to absolutely no fault of yours, and you have to do whatever is within your control to avoid adding further stress to your own life. This might involve reducing hours at work, taking on less projects with urgent deadlines or changing jobs altogether with a break in between. It's drastic, but it's for your own mental health and well-being of your children.
Based on our experience, the situation starts to improve (albeit quite slowly) after the second year. They will still get sick every 3-4 weeks but the symptoms get increasingly milder. I was originally hoping to work FT when DD went to nursery but that was physically impossible due to the amount of illnesses she got and also passed to me. I'm self employed so I was forced to cut back on projects & hours to the absolute minimum. It's ok as I wouldn't have wanted it another way but it's been a humbling experience.