So sorry to hear about you losing little Fern, I know it's hard not to feel guilty but you did the best you could for her - it's just a shame the vet couldn't find what was wrong quickly enough.
I am so deeply sorry to hear about you little on passing. We always second guess ourselves when we opt for euthanization... but please don't. It is never wrong to care enough to let them go peacefully rather then suffer. My heart and prayers are with you at this time.