Don't beat yourself up, accidents happen. I did a couple years ago with a day old chick and the big waterer. You live and learn. I guarantee you will never do that again.
I'm so sorry...It is understandable to be upset. It was an accident though, so don't beat yourself up too much. You love your chickens and this was not on purpose, she had an owner who loved her and you gave her a good life. I hope you do find peace with this, just give yourself some time. And hugs
but don't be upset, accidents happen. Last spring I had my broody out snuggling her and playing with her 3 day old chicks. When I was done I stood up, took a step and stepped right on a chick. It was horrible, I think she and I were both heartbroken. She eventually forgave me though, and this year I've gone back to cuddling my broody and her 8 babies. I'm just a little more careful where I step now.
I am sorry, I too have done that with a large waterer though
.
I know this hasnt' been mentioned, did you quarantine your new birds? you shouldn't add any new birds without separating them for at least 30 days.....it protects your existing birds