Thank you for being kind enough to do the right thing for this bunny. Please don't feel as though you have let her down in any way. Even with an appropriate antibiotic (you never did say which one you used, some are perfectly safe to use in rabbits and the exotics vets I work for that see rabbits (and have brought some back from dire straights) use antibiotics all the time but they have to be ones that are appropriate) from the way you describe her condition it seems that she would have had little hope for recovery even with stressful and painful treatment necessary. Even if you had all the money in the world, treating her would still have been a long and painful process that may well have ended in death for her no matter what. Ending her suffering quickly is the kindest thing you could have possibly done for her.
