I'm so sorry for the loss of your bunny. I lost my dog very recently who was perfectly healthy acting until that fateful afternoon when she refused her lunch. I took her in immediately to the vet who ran a bunch of tests, gave her an antibiotic injection and sent her home with me, stating they couldnt tell what was wrong with her. They didn't warn me she was going to pass that night. I watched her pass away, it was very hard, she was panicky and in pain, I couldn't pick her up (80 pounds) and put her in my van to rush her to the pet emergency. When she quieted in the early morning hours, I thought she was feeling better and fell asleep, She fell asleep alright, forever. I thought, too, she had a few good years left on her.
You did everything you could have done. It is hard and you will mourn the loss for a very long time. Grieving process is so hard! You just have to take it day by day.