BIG HUG
I am so sorry - depending on the strain of URI, you are really up against it, even with the best vet, meds and years of hand feeding experience! Some you simply can't win and they hit with terrifying speed. You are not the first member of the forum this has happened to, even though that is probably scant consolation tight now!
You did all you could, don't blame yourself, please!
RIP Bubbles and Fudge