Whoa...That's a little harsh.Don’t beat yourself up, rather use all of your pent-up anger at yourself to fortify your coop so that this can never happen again.
This might be an unpopular opinion, but just because you’ve read on BYC where severely injured birds were rehabilitated, doesn’t necessarily mean that should be tried. I know that chickens are extremely resilient and seem to heal miraculously, but that doesn’t mean they don’t feel extreme pain. IMHO, being “a fighter” doesn’t always mean they should be used as a guinea pig to see if they can be brought back from the dead.
I am sorry for your loss, but I’m glad your girl isn’t suffering anymore.