I know how easy it is to second guess yourself and feel like you didn't do enough, when you're looking back with hindsight. But when you're in the midst of the problem, things are much more complicated than that. All we can do is our best based on the information and help that we have at the time, and you certainly did that. But I know that it's hard to feel that truth in your heart when you're hurting so much from the loss.
And you really did do a remarkable job persisting in getting the diagnosis and care your rabbit needed, despite your vet giving you an incorrect diagnosis multiple times. Most pet owners would have gone with what their vet told them. You had no reason to think it was anything other than what the vet said, let alone a rare condition in rabbits. It was only your repeated insistence of getting to the bottom of things, that the true cause was actually found. Without your determination it may never have been discovered.
This allowed some treatment to be started and gave your bun a little more time with you. I know it's not as long as you hoped, but I strongly believe it's time you wouldn't have otherwise had with her if you hadn't been as persistent as you were with the vets instead of settling with their original diagnosis.
I'm so very sorry for your loss, and I hope time will ease the pain of your loss and you'll be able to find the comfort you need and deserve, of knowing you provided your bun with a good life full of love